I think you'd be hard pressed to make a non-scriptural moral argument against homosexuality. You could go the unnatural route, but that ends up looking a bit silly in my mind when you also have to consider men who choose to have sex with wombless women as evil too.
My favourite argument against homosexuality isn't moral but pragmatic. I first encountered it on a nice little Ku Klux Klan website a few years back. Its actually a standard claim, I learned subsequently, that is lifted and copied and pasted around the internet. I'm not sure where it started. It is (and I paraphrase because I don't want to visit it at work):
Semen contains Dangerous Things.
They give solemn stats, saying mouthfuls of semen contain enough viruses as can be found in pints of blood. Well, something like that, anyway.
It still makes me smile to think that this might be pursuasive to anyone.
But its still my favourite one.
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