This is a very ordinary, traditional Christian view with pretty strong Biblical support. It's also a view that has a lot of philosophical support, and not just from Christian or religious philosophers: Kant takes exactly the same view in the Metaphysics of Morals, 6:277-8, all the way down to asserting that the issue is one of a violation of logic (or basic rationality).
Sexual union (commercium sexuale) is the reciprocal use that one human being makes of the sexual organs and capacities of another.... This is either a natural use (by which procreation of the same kind is possible) or an unnatural use, and unnatural use takes place with a person of the same sex or with an animal of a nonhuman species. Since such transgression of laws, called unnatural (crimina carnis contra naturam) or also unmentionable vices, do wrong to humanity in our own person, there are no limitations or exceptions whatsoever that can save them from being repudiated completely.In the next paragraph, Kant goes on to define marriage as "the union of two persons of different sexes."
You're not obligated to be a Kantian, and I'm not one; you're not obligated to be a Christian either. But it's extraordinary to treat this as if it were a mere expression of hate. Kant, for example, has an argument for what it means to 'respect the humanity in one's own person' that applies here as elsewhere.
Kant is too important to the Left for him to be disappeared. I won't be surprised, though, if it becomes increasingly hard to find copies of his book that don't redact those paragraphs.