For one, I think very, very few people can pull of an open relationship unless there's an absolute state of trust between the two of you. Don't want to sound too critical, but what were you doing going to this girl's apt in the first place? The only valid explanation that I can see is if you wanted to test your man to see if he's true to you, in which case he clearly failed you when he started foreplay with another chick right behind your (sleeping) back. I'm not perfect by any standards, but that's pretty low. That's some shit guys can brag about to their friends about a girl they dont' really give a fuck about... Either way, I hope everything works out for the best for you. And even if you end up breaking up with this guy, just remember that it's better that it ended early rather than later when you would've been involved with him even more seriously.