Aww, poor you.  You can't even make it past the first thrust without squirting all over the place uncontrollably.  (Do this in a cutesy comforting voice)

It's okay baby, it's not your fault that your penis is too small to please me.  You should feel relieved.  Now that I'm fucking your friend, you don't have to worry about how you can't make me cum.

We could totally hear you beating off last night while we were fucking.  It's too bad that you can't get any pussy, but at least you get to hear your sister have an orgasm while you stain your sheets.
(Do this in a mocking tone)