They dont want to believe that there are some things beyond their (or their God's) control. They also see it as a slippery slope. If being gay is without choice, then being bi is without choice, then being a rapist is without choice, then being a pedophile is without choice, etc etc. I dont think there is anything wrong with being gay or bi because it hurts no one. The other things I mention should be prosecuted and if convicted the death penalty given. JMO
Religion is the reason people say homosexuality is chosen. When ever there is legislation regarding gay rights, who makes up the vanguard of the anti-homosexual movement? Religious institutions. And it is the influence of religion which has poisoned the well against homosexuals such that the very few who object to homosexuals on purportedly non-religious grounds do so because a religion-based society has taught them to consider homosexuality a perversion. We are finally starting to see this poison being neutralized by education, and exposing the related religious teachings as the product of bronze-age sensibilities.
Because they aren't us. If I had a choice, I would of been a heterosexual. Mainly because it would have made my life so much easier. The amount of hate gays get in some areas is just crazy (thankfully, my area is perfectly fine with gays). It wouldn't make sense if it was a choice. No one would be gay. Also, many religions preach that it's a choice.