And I always thought that when you have a girlfriend, she should be all the girl you need.
I think that's a pretty common belief - especially in American culture where that type of all-consuming relationship is idealized.
The truth is, no one person can fulfill all of your social needs, and it's unfair to ask that. For most of us, we will continue to need other people to interact to meet our social, recreational and emotional needs. The need for beauty falls into that category as well.
Think of it like this - it's unrealistic to expect your girlfriend to be at her peak physical appearance at all times. She needs a break occasionally just like anyone else. You still have a need to appreciate beauty, however, so you can fulfill that in part by appreciating other beautiful women - just like many men fulfill their need to watch hockey by going out with their guy friends!