Well this isn't exactly sexile but still:
My roommate and I were at a friend's place, watching a movie. The movie ends, I'm getting ready to go home (the roommate drove, I had no car back then). The friend says goodnight and goes to her room.. I head for the door expecting my roommate to follow, except that he asks me to drive his car back to our place. Apparently secret stuff had been going on!
I wasn't too excited about this (didn't have a license on me, and it was a weekend night in a college town so cops were out there).. But I had to get back home so I drove anyway.
Well, that was the beginning of their relationship; I must say at first I felt a bit upset since it seemed like I had lost two good friends to each other.. But I got over it quickly, thankfully
.
Over a year later they're still together (and now engaged!) though living in separate countries.. I hope they get to live together soon!