That doesn't also go against homosexuality. Just as there is no argument for homosexuals that also is for incest. Now granted I disagree with both, but think about it. "Just let them love who they want it doesn't affect you"
"It's their choice"
[spoiler]Food for thought[/spoiler]
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I think the most important argument for incest is the biological/morality aspect of it. I think the most split argument for homosexuality is usually the "born or by choice" i've even seen homosexual couples themselves feel different about it.