Well, you have to understand that a lot of these people take their "facts" from deranged preachers and the stupidity of Paul.
Romans 1:26-27: "
For this cause God gave them up unto vile affections: for even their women did change the natural use into that which is against nature: And likewise also the men, leaving the natural use of the woman, burned in their lust one toward another; men with men working that which is unseemly, and receiving in themselves that recompence of their error which was meet."
It's the couple of verses which probably did the most harm, as well as the only place in the bible that even mentions lesbianism at all. It presents homosexuality as some kind of divine punishment for not being faithful (enough.)
It's not as much forcing yourself to like guys. Just, you know, you skip your prayers a few days, and suddenly you start liking guys and knowing what colours go well with a fuchsia shirt, see?
You can probably see how those guys aren't going to be all tolerant of that, when their BS book says you could jolly well just have more faith instead.
Also, well, another part of the answer you'll start to understand when you read some of the bigotted messages against it. To a lot of people it's just a part of a surprisingly linear spectrum of fetishes and perversions. Sorta one day you start trying different positions with the missus 'cause missionary is so old hat, then you try something with oral sex and dildoes 'cause even cowgirl is old hat and boring now, then you start paying a dominatrix, maybe a bit of swinging and a threesome or two on the way down, then you start having sex with guys 'cause women are old hat and boring. And then you end up raping children 'cause now both adult genders are old hat and boring.
Which not only is offering them a ground to be against it as it is, but also creates the fear that one of those will teach their children that such perversions are OK. They can practically imagine little Timmy listening to a gay neighbour and going, "wait, so I could skip courting that moody bitch at school and just get to the really kinky parts involving three guys and a goat? Sign me up!"