Twilight wandered the halls of her vaunted friendship school. She levitated a checklist in her unicorn aura as she wandered.

“No trash spotted on the floors.” She checked a box on her checklist. With a nod, she continued onward through the school until she arrived at two doors one marked for stallions and the other for mares. With a deep breath, Twilight went into the stallion marked room and stood just outside the entrance. She twitched her ear to listen into the room and once she was confident that nopony was there she went inside. The room in question was a large communal shower room. The showers in back were communal but all the ones up front had curtains for privacy. With a flash of magic, Twilight turned on all the showers. She approached each shower and touched the water with one of her forehooves. Once finished, she turned all the showers back off. Next, Twilight wandered the room inspecting the floor for any trash or serious damage.

“Men's shower room in proper working order.” She checked off another box on her checklist and went into the women's shower room. Before she entered, however, she heard the sounds of some showers on. There were students there taking a late night shower. She decided to come back later but stopped when she heard Silverstream speaking.

“So Yona, did you smash anything good today?”

Twilight couldn't help but eavesdrop on what they were saying. After all, her school's primary focus was to sow a seed of friendship in other species.

“Yona not do much smashing today, but Sandbar sure loved smashing Yona last night.”

Tears welled up in Twilight's eyes. Sandbar was hitting Yona?! But they were such good friends. How was this possible?!

Silverstream's voice echoed. “Wait? I'm confused?”

Yona chuckled. “Hippogryph misunderstand. This joyful smash, that boy and girl do when they like eachother very much.”

Silverstream spoke up. “Ohhh now I understand. So how was it? Go on, tell me everything.”

Yona sighed, “Sandbar not good smasher but Yona forgive. After all, it pony and yak's first smash.

Twilight found herself blushing at hearing this. How did she not she forsee this happening sooner? She knew her students would establish bonds of friendship in her school, but she never expected they'd escalate this bond to romance. Twilight knew this was out of her depth. She'd never had any intimate contact with anypony aside from a crush she grew out of for Shining armor's right hand stallion. Wait? Shining armor.. that's it! Princess Cadence. Who better to lecture the students about love and romance? Twilight left the shower room, she had to send a letter.

-----

A few weeks had passed since Twilight overheard Yona and Silverstream talking about sex. She stood outside the school of friendship awaiting her special guest to arrive. Eventually, a shadow formed over her as Princess Cadence descended from the sky landing directly in front of her.

“Twilight.” Cadence smiled.

“Cadence.”

It wasn't long before Twilight and Cadence broke into their usual greeting. “Sunshine sunshine ladybugs awake. Clap your hooves and do a little shake.” Once they finished, the two hugged eachother.

“I'm delighted you could come Cadence.”

Cadence smiled, “and I'm always happy to help Twilight. Just show me to the lecture hall, and I'll do my best to educate your students.”

“Thanks Cadence.” Twilight led Cadence through the winding corridors of her school until they reached a door that opened up into a sprawling symposium. The entire hall was packed with the maturest students in her school including the foreign exchange students. Twilight did a proper headcount and nopony she invited to this lecture was absent.

“I'll introduce you Cadence then the class is all yours. I set aside plenty of time in their schedules so don't feel too rushed.”

Cadence nodded and Twilight entered the room. Everypony was talking with eachother. Most teachers would find this to be annoying and disruptive but not Twilight, after all how can you make friends if you don't ever talk to eachother. 
Alas Twilight called out to the class. “Ok everycreature quiet down.” Thankfully the students quickly quieted themselves and devoted their full attention to her. “Thank you. Today we have a very special guest speaker all the way from the Crystal empire. Please give a warm welcome to Princess Cadence.”

Cadence took that as her que to enter. “Good morning everypony.”

“She's here to talk to you about love. I'll leave you in her capable hooves.” Twilight soon left the lecture hall but stayed just outside the door to observe the student's reactions.

"Thank you Twilight." Cadence went up to the podium and cleared her throat.

“Ok then. Let's start with our genders but more importantly what makes our genders different from eachother. Now I know I'm going to be covering some pretty basic stuff, but it's always good to review the basics.” Cadence levitated the chalk started drawing a stallion's penis and ballsack from the side. Twilight could only stare wide eyed at the lewd drawing.

“Now I don't think I need to tell you what that is.”

“Oh oh. “Silverstream piped up. “It's a penis.”

Cadence nodded. “Correct, now there are many penises in the world, and most of them look like this normally. But if a male is interested in you it'll most likely look more like this.” Cadence promptly drew a huge erect horsecock on the blackboard just above the first drawing. “Now not all penises look like that. Ever since Equestria grew, we've been seeing all kinds of penises. Each one is pleasurable in their own unique way. I'd love to discuss this more, but we have a lot to cover. Now let's move on to female anatomy.” Cadence promptly drew a picture of a mare's pussy. “Now class what's this?”

“A vagina obviously.” Gallus rolled his eyes.
Cadence nodded. “Exactly. When a female is aroused, her vulva will get slippery and wet. Pony mares will also wink their vaginas like this.” She drew a picture of a mare pussy mid wink. “just like with penises, there are lots of different vaginas. Either way this is a bit of a refresher on the basic stuff now let's move onto foreplay.”

Twilight sank low to the ground. What the buck was Cadence doing? She wanted a romance lecture not sex ed. She desperately wanted to stop Cadence but knew she couldn't. It would be wrong to interrupt another's lecture, and it was her sister in law. Alas all Twilight could do is listen to Cadence's lecture in muted desperation.

“Overall caressing is the name of the game, you have to touch your partner in order to stimulate them. Now for males most of the sensitive spots are centered around the penis” Cadence pointed to the erect penis drawing with her ruler. “So remember ladies to give his dick special attention especially if he's half way between this state.” Cadence pointed a ruler at the un erect penis. “And this state.” She pointed at the erect penis. 

“For women it's a bit more complicated. Women like being caressed everywhere and everypony has a favorite spot. I know it can be difficult boys but you can do it. If your partner truly loves you, they'll forgive you if you take a long time finding their erogenous zones. If all else fails, go for the clit.” Cadence used her ruler to point to the clitoris she drew on the winking pussy.” This spot is guaranteed to get her motor going; but remember don't lean too much on this. Use it too much and it'll lose appeal. Now let's talk a bit about kissing. A hearty kiss is a great way to start any foreplay with your partner. It expresses love and trust and these are important in a relationship. Love is about finding what works with your partner and what doesn't. These are just general guidelines to get you started. Also remember there's no wrong way to kiss. Now let's get to what I'm building toward, sex.” Twilight cringed at hearing that word

“I'm sure it goes without saying that men are supposed to put their penis into a woman's vagina but remember a woman has many holes and sometimes she wants those other holes filled. For now though Lets focus on the vagina. Everypony pay attention because what I'm about to say is extremely important.” Cadence tapped her ruler on the podium for emphasis. 

“If you feel you're not ready to have a child with your partner yet drink a contraceptive potion before you have vaginal sex. This also applies if you're just hooking up with a stranger. There are unicorns who claim to have contraceptive magic but only trust this magic if you know your partner well. Never trust that your partner has drank the potion always drink it yourself. You never lose any pleasure from both parties drinking the potion. Keep in mind there are multiple types of potions, some last a long time and some only last a night. Some protect against STDs and others don't. Always remember to read the label on the potion you bought. As some personal advice I recommend you purchase these potions in bulk just after the estrus season ends, retailers tend to discount contraceptive potions. ”

Safe sex, thank Celestia Cadence covered that. Twilight sighed.
“Ok with that out of the way let's get started with vaginal sex. Now there are many sexual positions but we're just going to cover the basic standing mount and laying mount. A standing mount is when a stallion gets on a mare's back to penetrate her.” Cadence levitated her chalk and drew a crude picture of a mare being mounted by a stallion. “There's also the laying mount where a mare will lay on something comfortable, usually a bed and the stallion goes in over her like this.” Cadence drew a picture of a mare on her back with a stallion sticking his dick in directly above her.

“When a stallion mounts a mare, he'll often miss finding the mark his first few tries. Get ready for when he does ladies because you'll feel a huge surge of pleasure. Of course then the stallion will pump in and out of the mare. The more a stallion gets into rutting his mare the faster he'll pump. When he gets close to an orgasm he'll slow down before he orgasms. This isn't always pleasurable for the mare or stallion, however. Always remember to communicate with your partner. If you're not enjoying yourself, tell your partner how you feel. I know it can be hard because of the feelings going through your bodies, but communication is key to making the experience pleasurable. 

Mares can have multiple orgasms during sex, but Stallions usually only have about one or two in them if you're lucky. Boys, you can tell when a mare is climaxing if you feel her pussy try to clamp down on your penis or you feel a sudden spray of fluid on your penis. Ladies, when a stallion is getting close to his climax you'll feel his penis throb inside your vagina. It's pretty rare for a mare and stallion to climax together but when they do it's often a sign of sex done well. While we're correcting inaccuracies let's get this one out of the way. Most mares never moan like wild animals during sex. That's nothing but minotaur crap that clop filmmakers and authors put in their stories to convey mutual pleasure. If you truly want to know if you're pleasuring your mare or stallion just ask them.”

Twilight looked out over the crowd of her students. Just about everypony was eagerly watching Cadence talk perhaps too eagerly. Even Gallus, who would often feign boredom with the subject mater, was standing at full attention.

“Now let's get to what happens after the stallion climaxes. The experience of sex can be extremely draining for both parties. Seamen will almost always leak from a mare's pussy so be prepared to clean up the mess after. Remember lots of ponies — especially mares — like cuddling after sex so make sure you stay with your partner for as long as you can. And that about does it for everything I prepared. To conclude this lecture I'd like to open up a question and answer segment. Don't be afraid everypony, we're all friends here and I promise to be honest if you will.”

Almost immediately, several hooves and hands raised up.

“Let's start with the pink hippogryph up front.” Cadence pointed to Silverstream.

“I have a question... is it possible for a mare to love a mare in the way a stallion loves a mare?”

Cadence giggled. “Absolutely dear. I know lots of stallions who love other stallions and mares who love other mares. I even know somepony who loves both genders. And much of what I told you about caressing your lover works with same sex couples too. In fact, I find that same sex couples have better sex simply because they know more about getting each other aroused. The only substantial differences are what you do for the intercourse. For stallions they also have holes that they want filled most stallions prefer the asshole. As for mares there are all kinds of ways to be intimate from sex toys to just rubbing your vaginas together. Oh, I forgot about sex toys.”

Cadence cleared her throat. “Sex toys are often a fun accessory for loving another pony. There are rubber toys made specifically for pleasure but in a pinch just about anything can be used. Just remember to wash your toys off before and after you use them. Who's next? How about the blue gryphon.”

Gallus got up from his slump, “I don't have anypony I can be intimate with but I still get erections. What should I do about that?”

Cadence smiled, “In a pinch you can just caress yourself. This is called masturbation and it's perfectly healthy for everyone to do on a fairly regular basis. Repressing a sexual urge is never a healthy thing to do. It drives some mares and stallions to force sex on an unwilling partner. Remember everypony, consent is important for both mares and stallions. How about the yak in back.”

“Umm Yona have sex with somepony. It hurt Yona to have stallion penetrate her. Yona bled for several minutes afterwards. Is this normal?”
Cadence smiled, “first time?” Yona nodded. “That's perfectly normal Yona. When a mare is born, she has skin inside her vagina to keep out infections while she's young called a hymen. When a stallion first penetrates her he will tear this skin; back in my day they called it poping the cherry. It's almost always a painful and bloody experience for the woman but the pain passes. Having your cherry popped is a symbol of losing your virginity so try to enjoy it like I did. I remember when Shining armor popped my cherry.” Cadence stopped for a few seconds before continuing. “Popped my cherry in Princess Celestia's bedroom.”

Twilight clasped her hooves to her mouth. Seriously in her bedroom, does she know?
Thankfully Ocellus posed the question after she was called on. “Umm, does Celestia know you had sex in her bedroom?”

Cadence giggled. “Of course she does, not a lot escapes my aunt. She was ok with the whole situation, apparently it happens a lot.”

“I have another question.” Ocellus continued. “Changelings reproduce somewhat differently than ponies.”

“Oh.” Cadence smiled. “Would you mind sharing with the class?”

Ocellus nodded. “Sure. You see, Male changelings produce eggs in their bodies. When he has sex with a changeling female, he'll deposit his eggs into her womb to incubate. My question Princess Cadence is: if a male changeling wants to lay eggs in a non pony female would that be ok?”

Cadence nodded. “As long as she consents to it first. In fact, I know several mares and even stallions that would find themselves drawn to something like that. Thank you Ocellus you've taught me something today.”

Ocellus smiled. “You're welcome.”

“Right one more question left it seems, you pony.”

A random mare stood up. “What happens if a couple gets bored of having sex? Do they like, divorce or something?”

Cadence chuckled. “Of course not silly but I'm glad you brought that up. Every so often things can get stale with old-fashioned sex. And when that starts happening, it's time to spice things up with a kink.” Cadence smirked. “Now then let's talk about kinks because there are all kinds of kinks out there. In fact anything could be a kink to the right pony. Take what Ocellus just said about how changelings reproduce. To a non changeling that could easily be a kink they enjoy. The most common kink I encounter however is what's called bdsm in the sex world, that's short for bondage domination and submission. Basically what ponies do is that one pony in the pair becomes the absolute master of the other pony. The boss pony or as it's called the dom hurts the other pony often called the sub.

Yona cocked an eyebrow. “How can pain be pleasurable?”
“Nopony is completely sure why ponies find pain so pleasurable. Some scholars I read back in Canterlot suggest that the reversal of roles in bdsm is what is pleasurable. Just remember when doing a kink that both parties consent. And when doing bdsm always have a safe word so the dom knows when he or she is hurting the sub too much. Sometimes a kink might be too extreme for a stale relationship, though. In that case I suggest some simple roleplaying. Just remember to try things and see what works. I see we have on more question, after that we'll call it a lecture.”

Twilight took a deep breath, “she wasn't expecting this to happen but overall it wasn't a terrible lecture. The bdsm stuff was a little too extreme but Twilight felt that Cadence did a good enough job with it.” Then Sandbar opened his big-fat mouth. “I'm kinda curious princess; what kinks do you have?”

Cadence giggled. “Personally, Shining armor just loves it when ' mistresss cadey' gives him a good spanking” Cadence slapped the ruler on the podium with a seductive grin on her face. “He also likes it when I use some green changeling extract to pretend to be Twilight or his mother the taboo of the incest turns him on.”

Twilight's pupils shrank, and her mane stood on end. Her own brother fantasizes about bucking her!

“As for myself, I like to use my magic to swap our bodies. It's given me lots of insight into what turns stallions on over the years. Shiney appreciates it when I touch his sensitive teats in my body. I also use my body swapping spell to put Shining armor into a mare's body for some lesbian fun from time to time. And those are the kinks I'm into. Although we do try everything no matter how crazy it might be.”

Cadence sighed, “well that's about it I'm afraid. So in summation remember: consent is important, communicate with your lover, be honest, and don't feel ashamed of yourself, or your body have fun everypony, you're dismissed.”

As soon as Cadence said that, everyone filed out of the classroom. Cadence erased the chalkboard and went out to see Twilight curled up in the corner of the doorway. She took in her sister in law for a few minutes with a puzzled look on her face. “Umm Twilight? Are you ok?”

Twilight could only chuckle maniacally. “Ok. I'm perfectly ok Cadence... After all, I only just learned my brother wants to buck mom and me. And my sister in law just took a bunch of my students and gave them lurid details about how to get each other off. How could I not be fine?” Twilight felt her left eye twitch.

Cadence cocked an eyebrow. “But, I thought that's what you wanted? Your letter said that ponies in your school were hooking up, and you wanted me to educate them about love.”

“Love in the romantic sense Cadence, like the first date or flowers not spanking lessons.”

“I'm sorry Twilight. I assumed they already got past that part since they were already having sex.”
Twilight sighed. “Honestly Cadence, I could have lived with it if you had just left it at the bit about kinks. Buck it was a wonderful and informative lecture about the practical side of an intimate relationship. But did they absolutely need to know my brother fantasizes about me? Hell I didn't need to know that.”

Cadence blushed. “I'm sorry Twilight.”

“It's fine Cadence, really, we can't unring that bell...”

An uncomfortable silence descended on them. “Do you want me to give you some alone time?” Twilight nodded and Cadence left. Twilight meanwhile, picked herself off the floor and headed for the women's shower room. She needed a long cold shower after hearing about Shining armor's bedroom escapades. After that, she needed to get to work on writing apology letters to the parents of everypony in that lecture hall.
Bonus chapter
Several weeks had passed since Cadence's lecture at the school of friendship. By then Twilight had sent out apology letters to everypony's parents and heard back from most of them. Cadence was back in the Crystal empire that night in her bedroom with Shining armor sitting on the bed.

“I tucked in Flurry heart for the night.” Cadence turned around.

“Sooo sweetie... are you ready to talk about what happened at Twilight's school?”

Cadence grinned, “Yeah.”

“So, how bad was it? I know Twilight said you went a bit overboard talking about what we do in the bedroom and she's been giving me weird stares ever since. I can live with that but how are you holding up?”

Cadence giggled, “I'm feeling pretty good actually.” Cadence levitated a scroll and six letters from behind her back.

“What's that honey?” Shining pointed to the mass of paper.

“It's a letter from Twilight and the responses from the exchange student's parents. Twilight said that many the parents took the whole mess in stride a few were upset but I sent them letters of apology and they accepted. Thankfully nopony pulled their students out of the school over this.”

“What about the exchange students?”

Cadence grinned. “Somehow, the parents of the exchange students were the most satisfied. Want me to read what they said?”

Shining Armor nodded. Cadence cleared her throat as she levitated a letter up. “from grandpa Gruff: thanks lady for telling Gallus about the birds and the bees so I don't have to. Enclosed are five bits for your services.”

Shining armor chuckled “only five bits huh? You should get more.” 

“Oh Shining.” Cadence held up the next letter. “From Yona's mother Hilda: Hilda now know Yona dating pony. Hilda pleased with Yona's pick of future husband. Yona will give Hilda strong pony grandchildren. Pink pony make Yona better at smash, Hilda happy.”

Cadence went to the next letter. “from Sky beak: at first me and Ocean flow were mad at you putting these thoughts into our daughter's head. Yesterday however, our daughter informed us that she was gay and that your lecture gave her the courage to be honest with herself and others. We want to thank you for giving our daughter the courage to be true to herself. PS: where can I buy some of that changeling extract? I'm asking for a friend.”

Shining armor smiled. “It's always wonderful to hear that somepony had their life changed by my beautiful wife.”

Cadence went to the next letter, “from dragonlord Ember: You gave Smolder more education than most dragons get on sex keep up the good work.”

Cadence went to the next letter.“You might like this next one Shining. From Thorax: I'm grateful you expressed an enthusiastic interest in changeling anatomy. And your lecture taught Ocellus that love is about mutual respect. From the bottom of my heart, thank you Cadence for everything you've done for us. PS: would you and Shining armor like to try your body swapping spell with me? You might like making love as a changeling.”

Shining armor chuckled,“You said changelings deposit eggs in their women right? I've always wondered what it would be like to be a changeling.”

Cadence nodded and levitated up the final letter. “Me too, and here's the last letter from the mother of one of the exchange student's friends. From Palm shoals: I accept your apology for giving Sandbar a lecture on love and sex without asking me first. I don't really approve of everything you said about kinks but you mentioned safe sex. I'll settle for small victories. Next time though, please inform me before you tell my son anything inappropriate. PS: my little Sandbar told me he's got a girlfriend I'm so proud of him right now.”

Cadence set the mail down on a dresser behind her. Shining Armor grinned, “I'm delighted everything turned out ok sweetie. Since everything turned out ok. I'm letting you pick the fetish tonight.” 

Cadence grinned and lit up her horn with magic. The light from her horn grew and grew until the entire room was saturated in light. When the light dimmed down Shining armor's body smacked it's lips. “How about I get to be the husband tonight?”

Shining armor in Cadence's body smirked seductively, “it's always pleasurable to be in your body sweetie.”

Shining armor pounced Cadence and began making out with her on the bed.