The "everyone's bisexual" theory came out of the Kinsey research. The actual theory is that most people are to some degree bisexual, if you add up every dream, fantasy, experience, etc. While I agree that bisexuality is probably more prevalent than most people will admit, I also have some trouble with the broad definition. My therapist officially "diagnosed" me as swinging both ways, although I have never had an attraction to women. True, my earliest experiences were with girls, and when I was little I used to pretend that I was married to a girl, but that was because girls were who I had available to play with, and I was emulating the behavior that I normally saw among adult men.
As for hormones changing sexuality - I think it is actually possible. For me, personally, they only solidified my attraction to men. But while my sexual attraction hasn't changed, some of my attitudes toward sex have changed in subtle ways.
It's sort of like when you hit about 25 and you get that subtle shift in perception as the frontal lobe finishes developing. So many things that seemed oh so important to you before take a back seat, and things you didn't used to think about much take on new importance.
As a result of effects I've noticed while transitioning, I've come to the conclusion that hormones have a lot more effect on our attitudes and emotions than I ever realized before.
