The thing is, the way they dress and the nature of their bodies means that they’re all pretty casual about privacy. Sure, nobody wants to get seen on the toilet, and they’re all terrified of catching Splinter in the nude, but otherwise they’re pretty casual.
So when Leo’s in the shower, getting his share of warm water, it’s normal for him to grunt in affirmation when Mikey calls “hey bro” through the curtain. And when Mikey can’t find one of those weird lotion bottles Donnie produced from who knows where that he and Mikey swear by, it’s normal for him to push the curtain aside and reach in.
It’s not normal for Leo to hear the curtain move and freeze, the water splashing into his face.
It’s not normal for him to hear Miley mumble “scuse me” as he takes the bottle and goes back to the sink, and for his brain to turn it into, “Good morning, chunin.” 
It’s not normal for him to suddenly have trouble breathing.
The curtain shuts. He always shuts the curtain behind him. Mikey moves away–Leo knows this, knows that logically Mikey is using lotion and brushing his teeth and making gargling into a whole production as he wakes up. He knows, logically, that he’s safe here.
He knows this. But his body remembers hands running down his shell, pinching his thighs, caressing his ass. His body remembers being turned, looking up into Saki’s eyes as water dripped from the edge of the humans’ hair.
Something flirtatious, that Leo can’t hear over the sound of the water, the blood ringing in his ears, his own lungs struggling to function. The Shredder liked–likes?--his little jokes, always trying to make Leo giggle like a child.
His legs are nudged apart. A hand on his thigh, as light as a feather and heavy as an anvil. Shredder kisses him like someone in a romance movie, deep and passionate and unreal in a way that’s impossibly real. Leo lets out the faintest whine.
No-no-no. Leo collapses into a corner, hands over his mouth, trying to hide his frantic gasps for air. If Mikey hears this, he’ll try to help, even though Leo knows his brothers have to be sick of talking him through one panic attack after another. He’s a leader; he can’t have them wiping his tears and reminding him how to breathe all the time.
So he keeps himself quiet until Mikey walks out the door. Until he’s left with the ghost of Saki’s fingers scraping over his skin, turning him to face the wall again as he braces his hands and forehead against the tile.
Not real not real not real 
“So good for me.” 
The water rushing over them both. There was always warm water in the Foot Clan. There was always privacy, except where there wasn’t.
If anyone heard the sounds they made behind that curtain, they never said anything. If anyone saw Leo’s bruises and bite marks, he didn’t hear them mention it. If anyone ever put together the puzzle pieces of witch’s spells and a small soldier at a big man’s feet and the way Leo’s eyes sometimes went blank at Saki’s touch, they never dared put it into words.
Saki liked putting things into words. He always talked so much.
“So tight.” 
Sometimes he was sore, and tired from a long day. Sometimes he was just waking up and not in the mood. But he didn’t say anything, so that meant he agreed, right?
“That’s it. You need this, don’t you? You need a good hard dicking.” 
Sometimes he liked it a lot.
“That’s–nnh–it.” 
Tears spark in his eyes, hot and painful. They get swept away by the relentless water, pounding down on his flesh like bullets and words and memories.
By the time Raph yanks the curtain aside again, yelling about how Leo’s taking too long, he’s shaking too badly on the ground to flinch. His brother drops to his side, all concern and care and so many things Leo doesn’t deserve, and all of his explanations and apologies are crushed up into a useless sob.