I believe that if you wanted to you could become "unattracted" to someone if you really intended to, mostly by digging up enough negative qualities in them. But for the most part, it's just like you're favorite food. Everyone likes something different. Some people like blondes, some people like redheads. some people like big butts, others prefer a nice nose. Some people love carrots, others hate them. I feel the same type of thing goes along with gender preferences. Sure you can alter what you feel, but I believe that would again, be lying to yourself.
I like women. I'm sure if I pushed myself hard enough I could learn to hate them, or learn to like men....but the fact of the matter is.....I originally liked women, that was who I was before I went and forced something else upon myself. I have basically changed who I am.