Because namecalling will change someone's opinion. If you want to claim open-mindedness, then listen to what he has to say, rather than just flaming him.
Anyway, from the standpoint of a soon to be husband, and who plans on raising a family, I personally think that homosexuality is not something you're born into, but rather are made through the years. Psychological studies show that it's a mental thing. For people who believe in a complete moral system, it's immoral. You can't change a persons moral system by lashing out at them and calling them bigot, close-minded, and so forth. That's the language of a rebellious teenager, not an educated or open minded person. If people were truly open-minded, they would listen to everybody's opinion, rather than just not wanting to offend someone.
Brokeback Mountain had two men who had a relationship, got married to women, then still had sex with one another. That promotes cheating on a vow that used to be sacred. The whole movie reeks of a life that is constantly spent lying to someone that you made a vow to. It's not romance. It's deceit. Huge difference. So, the movie promotes cheating on your wife, and to make it worse, cheat with another man! THAT is the horrible part of it.
So, opinions on Brokeback Mountain isn't just about homosexuality. It's also going back on vows and promises for the sake of a moment. That is what is disgusting.