Comparing your girlfriend with other women is a very dangerous proposition. I'm not saying that i do it, but i would imagine that its very unhealthy when you compare what you have with what you think you might want more, or what you see is available elsewhere.
And worse yet, if your girlfriend knows that you're doing it, it can be very damaging psychologically. She begins to wonder whether she's your second choice: the reason why you're with her is because you can't be with another. She begins to think: i'm not good enough, i don't deserve him. And worse of all: i wish i could be her. Because she loves you, these thoughts will begin to creep into her mind each time you openly compare her with another.
The basis of the comparison is not important. You may be comparing her beauty with that of another. The other girl has longer legs than you. The other girl has more pretty eyes. Ooh... doesn't she have a shapely figure (sorry, yours cannot match it)? You may be comparing her intellect: why can't you be as smart as her? Or you may be comparing her ability to "be there for you": she's always there for me when i need her, why aren't you?
What's important, and very important to note, is that comparisons are always, always, without any exception whatsoever, BAD. By comparing, you're telling her she's not good enough; someone else is better. That's just plain wrong.
The fact is, there will always be someone taller, prettier, bustier, smarter, richer, more kind hearted, more understanding - that's a given. No matter how good we think we are, there is always someone who will be able to do it better. The point is that, once you've made a commitment, a commitment is about believing that there is no one better, no one above her.
* The above is a note to myself, a reminder. Once upon a time, a long time ago, i didn't believe. Now that i'm in love, fortunate to be so with a wonderful woman, i want to always believe.