John comes back from his army mates meeting very drunk. He manages to get to his bedroom on autopilot and almost immediately falls asleep half-undressed.

Later Sherlock enters his room for some reason (maybe he needs John for something, but realises that John is not available at the moment). John is sleeping soundly, but Sherlock notices that he's really hard in his sleep. He's been in love with John for a long time and John is the only one he's ever wanted physically.

He hates himself, but ends up climbing onto John's bed (maybe he's just curious at first and wants to touch John tentatively, but can't stop), slicking up his own virgin arse with something, then lowering himself onto John's hard penis and riding it until he (or both of them) comes.

Graphic stuff is very welcome. Please, very guilty Sherlock, enjoying every moment of having John's prick inside his (no longer) virginal bum.

Maybe John is half-awake through some of it or thinks that he's dreaming. Maybe he learns that it wasn't a dream and he's furious and/or turned on. I don't really care how it goes.

Happy johnlock in the end is perfectly okay, but maybe John never learns what happened and Sherlock just keeps pining quietly.