Dude your fucking stupid. Seriously calm the fuck down, this dude didn't know her or and you didn't know that she had a boyfriend. He was making a friendly gesture and you and your stupid friend freak the fuck out.

And what the fuck you gonna do, beat some random dudes ass because he kissed your girlfriend on the cheek??? Its not like he raped her or anything.

Guys like you disgust me. You think you are all hard or some shit and get all pissed off and try to go "looking for some guy to beat his ass" when he did nothing wrong.

In the end this girl (if she had any brain at all) will leave your loser ass because one day she is gonna have a real job and "friends/co-workers" that are going to be guys, and shes not gonna be able to live any sort of normal life because your ass if going to be freaking out because she talked to some guy or shook his hand. You need to calm the fuck down because this girl isn't gonna stand for this shit forever.

I have seriously seen my girlfriend kiss my roommate in front of my own face, hell I told her too. It wasn't a big deal and it was all fun, but I know that at the end of the day she is going to be sleeping with me and she will always love me. Learn to trust your woman, damn your fucking dumb.