Honestly-I'd be a little weirded out for awhile and very confused. Disappointed and sad, feeling rather left out. Probably like you are.
Do understand a low sex drive can still hit moments of...peak...suddenly, it's just quite a bit more rare than higher drives. Given that it sounds like you still want to be part of this group, I'd suggest ignoring it until you feel up to talking to either about it. Obviously you're upset about it, and you know you're upset, but you don't want to lose what you had. So, spend that pent up energy in getting it back. Hang out with your friends, mend fences, build bridges, go and do stuff with them.
From all you say, this group of friends is important to you, despite what happened. You're angry at her and him for going behind your back... perhaps even more at her than him, which, if you'll understand, that's probably a remnant of the frat mentality. There's more to it than what you're seeing, and once you're ready to ask (out of curiousity not out of anger or disappointment), do so. If they're as accepting and understanding as you think, they'll answer.
I can't say I agree with your ultimatum about sex in a relationship-especially since stress can drive even the strongest sex drives to almost nothing (and with her school work it sounds like that may have happened), but I don't know how old you are. Sex has different meanings and yes, age does influence it, not to mention the definition of "relationship". It doesn't sound like it was something serious you were after, otherwise the relationship would have been less centered around sex and more around the person and having them around. That's not a judgement, just an observation.
All in all, your friendships seem more important than any of the rest of this, so I'd suggest concentrating on that first and the rest will fall into place, be it via time and perspective, talk, or just happenstance. If you really need to blow off some steam, may I suggest trashing some boxes in an empty car lot? Throwing glass bottles or cheap dollar store plates? Smashing stuff often helps, and you can't always be fortunate enough to have to help renovate-tearing down walls with a sledgehammer (highly therapeutic).