So no one here has never known a switch-hitter? Plenty of people go gay and go straight, preference is not always biological (I know everyone here has seen sexual selection as a form of sociality). Hell, they can switch back all the time whenever they want. What most however want to do is pigeonhole their preference to create a fine line between a preference that just doesn't exist. Most would be surprised with themselves if they actually turned off their inhibition switch... just saying.
Make a note however, I am not saying sexual preference is a conscious decision. It can be if people so desire something. That being said, there are people who have a disposition (that is more than likely moderate when discerning influence; i.e. natural disposition or nurture) towards the same sex that is wholly natural (is not a freak abomination/accident of nature) and biologically driven. It's been shown that males may have a
prenatal disposition to homosexuality based on how many older male siblings they may have. This particular article is from 2006, so there is no doubt more information on the subject now that I will have to catch up with.