There are plenty of things that people do every day that contribute to the potential spreading of diseases, from every kind of sex to not wearing a mask when you’re sick.

To single out anal sex as a sign that homosexuality is immoral (despite the fact that vaginal sex can also spread diseases, and despite the fact that anal sex is not exclusive to gay people) is a sign that the person you’re talking to is biased and arguing in bad faith.

Ethically speaking, if someone wants to live by a moral system that differentiates between right and wrong based on the potential to spread disease, then that’s fine, but that logic still needs to be coherent and apply to all things, not just selectively to things that they dislike.

But anyway, if they’re sophists, you probably aren’t going to convince them. If you have to engage with that shit, then your best bet is probably the socratic method: ask them targeted questions to poke holes in their flawed logic until they back themselves into a corner. You know what they’re saying doesn’t make any sense, so simply asking them questions which reveal more contradictions will force them to adjust or abandon their position.