I want to know the reason, why exactly is it unmoral and why does it justify homophobia?
Because you're confirming their prejudices involving the notion that homosexual men will fuck anything that makes their dick hard — even their own fathers — and that the "gay agenda" is a stepping stone to normalizing things like insect and bestiality.
Meanwhile straight men will rape their daughters like it's nothing
Heteros are so fucking full of shit it's disgusting, they're so prejudiced that the things they think The Other does are gross, but when they themselves do it it's perfectly fine
Just like Christians and Republicans lol.
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That being said, I personally don't see anything morally wrong with incestuous relationships, as long as it's not producing offspring. Realistically speaking, we're all products of incest - there's no other way that our species could have survived. The only reason it's looked down upon now is because it's bad for the gene pool, and also because it's seen as an "antiquated" practice.
You know that it's bad if you talk like all heterosexuals are bad. I think we all need to respect others if we want respect from others. Also that is also a bad way to say it, just because someone does something bad, it doesn't make it right you to do that too. I'm sorry, but that's how i see it. Also i want to point out that gays and heteros are same. Every human person is equal and deserve to be respected, but if that person does something horrible, they can't be respected. Respect is earned and not given just like nothing and opinions need to be respected too.
Also lastly that about we all are products of incest, it doesn't mean we need or can continue doing that. Something that was okay in far far away in past doesn't mean it's okay these days.