Okay....
Here is MY confusion.
She has a possessive boyfriend
but she brought YOU upstairs to meet him where the four of you cuddly drank beers, at which time he left his girlfriend alone with the guy who she brought home with her?!? And then, you all went for breakfast the next day?!? Didn't he wonder what you were still doing there and why you woke up cuddling on the couch with his girlfriend?!?
either way, you're in a crappy situation. i hate people who play games. she's obviously toying with your emotions. i guess the issue is whether or not you're cool with that, because you obviously know it. we can't really give you advice on that one
Go with what feels right. and i mean
emotionally 
.