Aimster: dude that really is not cool. i would not put up with that crap. i have kind of had situations similiar to yours as far as the 'picking up girls at the club when already having a girlfriend' type deal.
i have only had 2 girlfriends, and they have both been pretty serious. i am currently 22, and i have been in my current relationship 'seriously' since last summer. i had been seeing the girl for about a year before we became serious. all the time, when with this girl and my ex (which I was with since i was 17), whenever we would go to the clubs my friends would ALWAYS try to get me to hookup with other girls, which again, is not cool to me, him knowing that I have a gf already and knowing how serious it is. the thing is, my friends don't have gf's. they are the type who like to go out and hook up with random girls at the club. i am not like that, at all. i like having 1 special person that i can be with. whenever my friends are like 'yea i hooked up with this chick at the club' or 'i had sex with this girl the other night' i am just like 'and you are telling me this why? when are you guys gonna start dating? etc.'
i think the main thing my friends try to get me to hook up with the girls is because (and i'm not trying to sound like an arrogant fvck) is because I am the one in the bunch who is the best looking, and they see me getting girls left and right when we go out, without even having to try. the girls just come up to me and talk to me and tell me how hot I am and what not. i honestly think they are just jealous because they can't get em that easily, and that they want me to take advantage of the situation because they can't. i kinda of like the feeling just KNOWING i could have those chicks, but not actually getting them.
one time my best friend and I were talking, and he was saying to me 'how can you have sex with the same girl all the time? i can't do that, i get bored' and my reply was 'i bet you i have had sex more times this week than you have had all year' and he kinda chuckled and was like 'yea, but it's with the same girl.'
the thing that pissed me off was he was saying it like it was a bad thing. my manager at work was also right there and was kinda like 'wtf... who cares, i have a girl too and its great.'
just remember this bro (but seems like you aren't gonna have to worry about this) ... an hour of pleasure with some random hot chick is not worth the time you have spent with your current GF, no matter who it is.