Because they're also gay but can't accept it

Sometimes it's "reaction formation". They like a thing. That makes them uncomfortable. They instead go hard in the other direction - they hate the thing it's horrible and bad and the worst.

Sometimes they're just... basic, and need the world to fit into simple boxes. Anything else stresses their brain.

Because some ancient sex manuals written by ancient goat herders said that gay sex is gross, but it also said love thy neighbor, and none of the peasants at that time had the wherewithal to think "hey maybe only one of those things actually makes sense"

Edit: I'm not giving the peasants enough credit - a bunch of them probably did have the wherewithal, but the people in power murdered them for questioning the ancient sex manual, so that stopped that pretty quick

Amen to this! Let people be, let people fuck what ever they want.