I don't think you are. As a point of reference, I am a lesbian and I occasionally have fantasies of men. When I actually think about sleeping with one - and I did attempt it in person once - it doesn't appeal to me in the slightest. I think you are well within the straight side of things; a little attraction to your own gender is not uncommon but not talked about. It doesn't mean anything.
Also I'm pretty sure I've heard of other people having obsessive thoughts about this subject. I used to about being straight! It's definitely influenced by OCD if nothing else.