“Why are Christians homophobic?”
Homophobia is a ridiculous pejorative that applies literally to almost no Christians in the real world. It fails miserably as language, in that if it means anything at all, it means “fear of that which is the same”. The term is a convenient way to deflect arguments that address the dangers and evils of a lifestyle that exalts sodomy. I do not use it.
That said, if you don’t celebrate perversion, anyone who inquires why that might be will probably use the word, so we may as well formulate an answer for them.