I definitely think about this from time to time and has been a topic on my mind a lot at certain points during my current 5 year relationship. I often kind of find a middle ground, because what I like about these girls the most is that they are cool and I enjoy being around them. We become friends and what I often find out is that if we were to date, or even just have a sexual relationship, it would not be as good as what I have now.
There was this one girl that was my best friend in colleges gf. they broke up and we both live in the same city now (she just moved to mexico though for a few months). She beautiful, an artist, rebel, witty the whole package but as I got to know her, she is kind of crazy, insecure and just not into the idea of sticking around for anyone too long. She became good friends with both my girlfriend and I. I think becoming friends often is even better than chasing her in any way especially if you have some amazing going for you. Who doesn't love beautiful company? Though I may be chasing girls here and there, in the end, its just a reminder that I who I am with is incredible. She does the same thing to some extent. Its part of being young and in your 20's (especially in a major city like we are). No harm, but no cheating.
I like to crush, who doesn't? But in the end, I am way more happy with who I am with. The grass is always greener on the other side, as they say.