I personally wouldn't want my girlfriend out partying with a bunch of "guy friends" in the first place. I've never met a chick who has "guy friends" that their "friends" didn't want to fuck the shit out of her unless she was really ugly and fat or something. It's pretty amazing to me how naive women are when it comes to the relationships between men and women. Unless the guy is either gay or has an extreme amount of self-control, he's probably going to want to nail his female friend (assuming she's hot). That's just the way it is.
So if I were your boyfriend, I'd be questioning your actions regardless of who is technically at fault. Having said that, it may seem like I'm coming down on you to try and make you feel guilty, which I'm not. But at the same time, you really shouldn't be putting yourself into situations like this, in my opinion.
Either way, the culprit is a major sleaze bag.