Argument: Legality of incest cannot rely on whether couples reproduce
Adam Graham. "Are you ready for incest?". Renew America. October 28, 2004 - A major argument against incest is genetics. If you allow two closely related people to marry, their offspring may be prone to some serious defects because he or she would have genetic defects and poor general health. However, this argument fails under the regime of gay marriage because many genetically flawed people who will have children that will develop deadly diseases are allowed to marry. In addition, with gay marriage, we say that reproductive potential between the marriage partners is irrelevant, so who are we to say that the potential of reproductive defects should stop people from getting married?
"Sexual Ethics: Consensual Incest". A Nadder. October 13th, 2008 - It causes birth defects -- finally a somewhat true statement (although apparently the best strategy is to actually marry your 3rd cousin). But if that's reason to outlaw incest we need to outlaw drinking/smoking during pregnancy (which we don't and it would be a big deal to do this). We'd also need to prevent 2 carriers of a genetic diseas from procreating. Finally despite what you hear on the news, we're not living in the 12th century. Sex ≠ babies. 2 sisters having sex, or a heterosexual incestuous couple using contraception will demolish the argument.