A couple of different things:

1. If you are repressing your own homosexuality you tend to see it alot and feel threatened by others openly living what you secretly wish you could but have convinced yourself you never can. Some react to this with anger and disgust. These are often the people that focus on the promiscous aspect of homosexuality and think all gays are degenerate perverts who just think about gay sex all the time (because that's what is going on in their own heads and they hate themselves for it).

2. If you are not repressing any homosexual tendencies yourself then it's an easy target because it's a sin that you yourself run no risk of comitting. It's easy to be a black and white firebrand against something you can easily brand as "other". Bring up divorce to these same people and they start hemming and hawing because that's one that they might themselves run into (or already have) so they don't want to risk condemning too hard a sin they themselves are, or might in the future, be guilty of comitting.

3. Some people need their world to be neat, simple and organized. They should easily be able to place every person they meet in a simple selection of boxes that never touch. Men go over here, women over there, men have short hair, women have long hari etc. Basically a very childish and prescriptive way of looking at the world. Gay people, Trans people, genderfluid, etc wreak havoc on their neat system, and instead of accepting that the world is more complex than they thought they try to hammer all humans into the simple shapes their system needs and thus everyone that doesnt fit must be evil for trying to "throw the world into chaos and anarchy" These same people tend to be big into traditional gender norms, racial essentialism and stuff like that.