I think there are multiple reasons. Some are religious, so are just natural in leaning (gays cannot reproduce, therefore it's not what nature had intended), and others don't want to give into a special interest group claiming they deserve rights just because they choose to be gay.
Yea...about the interest group thing...I don't think being bi makes me special. Or at least no more special than any other person. I don't deserve any extra, or special, rights just for being of a different sexual orientation--that's crazy.