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 screws with the gene pool. Not to mention that it often happens under the age of consent. The child that is produced has a VERY high chance of genetic disorders, which are often (if not usually) severe. I believe women as well (not sure about men) have sort of a "natural defense mechanism" that prevents attraction to their siblings or parents. I'd have to look it up though. Incestual -blam!- happens as well. None of this happens with gay sex. STDs are a big problem, but they don't mess with the population at large. The price of loving who you love. Nevertheless, people use false "concern" as a political point, taking the STD numbers as an argument against homosexuality, all under the guise of concern.