I hate to say it because it's politically inconvenient, but I have to say that there does seem to me to be more fluidity about sexual orientation, especially for women, than many people claim.
But the thing is, even if change occurs over a lifetime, it doesn't mean that those changes are voluntary, and it doesn't mean that the person wasn't, say, genuinely bisexual for a period and now genuinely homosexual. without in any way wanting those changes to occur.
I hate it when people say racism is bad and homophobia is ok because you can change who you have sex with but you can't change your skin color. Every time I hear that, I wonder if they think that if it were possible to change skin color, all non white people should choose to do so.