As my Dad said to me before, "Looking's OK, it's touching that will get you into trouble". Go ahead and look, you're human aren't you?
I don't recommend blatantly staring right in front of your girlfriend, but we all get caught sometimes (including the girlfriends)
If my girl wants to check out a good looking guy, I really don't care (I'd be worried if she didn't actually)- after all, I'm the one who'll get all of the benefit (and vice versa)