Because they are trying mightily, with my tax dollars, to force their immoral and dangerous lifestyle choices on me, and to indoctrinate my children in it.
Get it?
Personally, I've worked in the entertainment business so consequently I know a few homosexuals. I don't condone their preferences, but neither do I feel the least bit threatened by them. Nor do I think the vast majority of them are pushing the radical agenda. Most of them simply wish to be left alone.
Now I know there may be some who are leftist radicals who push an unacceptable agenda, but let's not judge a whole group of people based on what the most radical and preposterous of them do. God forbid, if conservatives were judged by that standard the left could just as easily paint us all as David Duke and Pat Buchanan.
Get a little perspective won't you? Live and let live. Isn't that what being a free country is all about?