Because you're not totally straight. On the scale used in the Kinsey studies (1-6) very few people are 100% homosexual, or 6 on the Kinsey scale, as very few people are 100% exclusively heterosexual or 0 on the Kinsey scale. Most fall somewhere between 5: predominantly homosexual, only incidentally heterosexual and 1: predominantly heterosexual, only incidentally homosexual, Most people fall in the 2-3-4
Since you are predominantly heterosexual, you are usually only attracted to men who look female, except for what's between their legs. If this is something you want to explore past fantasy, I suggest you use the terms Tran sexual and/or Cross Dresser, as "she male" is a porn industry term and many effeminate men are offended by it.
It is estimated that 85% or more of boys and 65% or more of girls share same sex play during sexual development. Many go on to lead "normal" heterosexual lives while some decide that they prefer the same sex and some remain bisexual. Maybe you were one of these and enjoyed it, but your preference for woman brings out the T-girl fantasy.
Human sexuality is never set, and changes with age, experience and emotions. The world would be a much nicer place if there were no sexual labels and it didn't matter who consenting adults choose to share pleasure with, as long as no one gets hurt.
Just sexual. No other label(s) required.