[His face goes bright red from his neck to the roots of his hair and he sputters a bit.]

Sherlock, we're not even having-- we only had sex when we were a couple in an alternative reality. I don't think that counts as us having sex 'exclusively.'

...

And if that event truly put you off that much, I'm sorry. I didn't exactly have control over what our Hogwarts selves got up to. Sorry for the fact that my magical self enjoyed snogging your magical self, must have been quite awkward for you to deal with now that you're back to reality and thinking straight.

[This hurts, more then he'd care to admit, so he can't help being a bit clipped in response.]