Either John or Sherlock (who are not in a sexual relationship) has/develops sexsomnia (http://en.wikipedia.org/wiki/Sexomnia). I'll leave it up to Author
- whether Sherlock identifies as asexual, demisexual, sexual, or something else;
- how the other party reacts to the sufferer's advances (and lack of acknowledgment of what happened - because he doesn't remember these episodes);
- how they both react once it becomes clear what's been going on;
- and whether their relationship takes a turn for the better or worse.

Obviously this involves dubcon, but I'm fine with all-out noncon too.