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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Incest is a sexual act between family members. Homosexuality, like heterosexuality, is just that - a sexuality. Sex between family members has a good likelihood of creating a deformed baby; homosexuality does not.