As you can see I've said I'm 99%Gay and 1% straight, that's not because I conciously think of ever having sex with women, and I've never really been attracted to women, but I've had crushes on girls, especially when I was younger, and hed sex with one or two in the past aswell.
Despite saying I'm only 99%Gay I think in practice I'm probably 100% gay, I just admit to myself the possibility that one day some special lady might come along who'd sweep me off my feet and we'd ride (LOL) off into the great big hetero-sunset.
It's only a theoretical possibility mind, and not one I think is likely and certainly its highly improbable going on the evidence of my life so far, but love is a strange thing (as opposed to sex which is a pretty un-strange thing) and you can never be 100% sure what your emotions will end up making you do, in my opinion.
I'm not sure if it's possible to go from only (really) being attracted to one sex to being exclusively attracted to the other, that seems unlikely in light of how human sexuality seems to function, but I think it's possible to go from being only attracted to one sex and then falling completely in love with a member of the other and never really wanting to have relationships with anyone else. I suspect even in this situation though that you'd still be attracted to members of the sex you were originally attracted to, just not so much that you'd be willing to jeopardise you primary love relationship unless your actually bisexual and have never really considered the possibility and have placed too much emphasis on a relationship which was just evidence of a bisexual sexuality and not the love of your life.