I am a bit like this and it drives me crazy. I love my DP, we have been together for 16 years and I still fancy him and our sex life is great but I have always had one crush after another on other men. I got my first crush on a real live boy when I was about 12 and its been relentless ever since. If you knew me you would never guess I was so "boy crazy" but I am.
I think part of it is just that I do have quite a high sex drive and I can look at a man I don't even especially like and think I'd like to shag their brains out! This is normalish I think and doesn't disturb me too much but the epic crushes I get do upset me and shatter my peace of mind.
I'm very ashamed to say that in my early 20's when I was dating another man I did act on these crushes and cheated on my pervious boyfriend a number of times. The thing is that if any of these men ever actually wanted a relationship with them I ran a mile! The ones who blew hot and cold or rejected me would send me into over drive and I'd become highly limerent of them and would fantasize about them for years but only ever about the point where they fell in love with me.
Like a previous poster said, I have low self-esteem and I seek validation from men. In my fantasies my main thrill comes from imaging their desire for me. Its pathetic actually but this fantasy gives me both an imaginary sense of validation and the emotional thrill and high of early falling in love, even if it isn't real.
It could be seen as harmless but I think it does erode my peace of mind, it distracts me from my other goals and it is a for of disrespect to my partner because there are times when I am with him, having a lovely time but secretly imagining that I'm with someone else, which is so unfair, especially when it is DP giving me said lovely time!
In some ways it is easier now as I work from home and don't meet as many men but I still manage to find a man to fixate on, a celebrity (Cillian Murphy!) or I dredge up some old crush most recently an old university flame who broke my heart and has surfaced on facebook (he lives overseas now so not likely to meet him again ever but he's provided useful fantasy material).
I used to wonder if their might be more to these feelings of mine, beyond emotional masturbation but I really don't think there is anything meaningful about them e.g. I don't really believe any of these men are my soulmate or that I am missing out. My evidence for this is that when I first met a close male friend of mine about 10 years ago I developed a major crush on him and was sorely tempted by him although I managed to restrain myself. We did stay friends and now 10 years later while I like him as a friend I realise that ending up with him would have been a mistake and that we are not at all compatible, we wouldn't have had the lovely relationship I have with my DP. He is a bit of an ego maniac, he has a drinking problem, he can be cold and superior he needs to be the centre of attention all fine in a friend but not fun in a partner.
I realise I am very lucky to have what I have with my current partner, we are very much on the same wave length, like the same lifestyle and have a lot in common, he is supportive and kind, the sex is great and we still fancy each other. All the other stuff is just as I said emotional masturbation and escapism. I am bored with my work and spend hours online rather than reading books or doing other things that make me happy like doing art etc. These are the things that would the real validation I look for but they all require more work than lying back and closing my eyes!