Okay, let me break this shit down:
It's normal to be attracted to other people, to want to fuck people beside your girlfriend. It's even normal to experience lust for someone else far more than for your current partner, who you've been together with for some time, I'd assume.
Far more important than these things is the way you look at them. Of course people you haven't fucked yet/made out with yet have that shiny shine of novelty about them, that charm of uncertainty and the yet unexplored. But you should realize that it's that. If you're with someone, you learn to value them and who they are, and you start to see things about them that others simply don't have. It's not necessarily always a great ass (could be, though), but there's other things to value about someone.
Lastly, if you really don't feel like your girl is that attractive, then that ultimately doesn't have anything to do with other girls. That really is just not feeling attracted to her enough. If you're with someone, and if you love someone, you should think they're beautiful and you should enjoy their touch and being with them. Not necessarily every second of every day, but you should genuinely experience them as a beautiful being who you spend your time with for a reason. Because if you don't, why bother?
And lastly +1 (cough):
Most people in a relationship experience regular sex and intimacy and then begin to take those as natural facts of life that just kind of happen to you on a normal basis. They forget that all these hot bodies they see in the streets are not theirs to take, that these uncountable hot women may not jump at the chance to get into bed with them, and that overall, the love and affection and validation they experienced by having a romantic relationship will end brutally and suddenly once they are out of that relationship. So don't fucking take things for granted.
