well to a point, at least in my eyes, your girlfriend to a point should also be able to take care of herself - you can't exactly see a peck on the cheek coming, sometimes you just HAVE to let it slide - if you see the guy, just pull him aside and be like " hey man, I don't really appreciate you coming onto my girlfriend last night, try not to do it again" and just let it go - if you trust your girlfriend, and know that she can take care of herself and handle herself in a situation like that again, then your good to go

but if he gets in your face about it, you sock that motherfucker and then you curb stomp his ass