From a woman's point of view, I have been married to my husband for 20 years, I love him very much, and our sex life is GREAT. Unfortunately, within the last few years, I have to admit that men are looking very good to me and I feel like a cat in heat. I have been with my husband since I was 16 and yah...I wonder what it would be like with other men. So back to your questions...I think you will probably always have these feelings, it's how to control them. Unfortunately, I haven't figured it out yet. That is probably why my marriage is in jeopardy.
Maybe your relationship with your girlfriend isn't marriage material, but just a great friendship.