"As Frame Game found in the case of bestiality, the best argument against a questionable practice comes, inadvertently, from within it. In this case, the answer comes from Aunt Kate's advice column at cousincouples.com. "I tell almost ALL my correspondents who are considering expressing a more than casual affection for their cousin to remember a few important things," she writes. "The first one is that you already have a guaranteed, life long relationship that you will live with for a very long time. Don't mess it up."
This is the problem with sleeping with your cousin. You can move on from an ex-spouse or ex-lover, but there's no such thing as an ex-cousin. How are your parents and your ex's parents supposed to handle a nasty divorce or a breakup? How can they support their kids without antagonizing their siblings and their siblings' kids? You've wrecked your whole family. It isn't as bad as if you'd slept with a sibling, but it's a lot worse than if you'd slept with a friend or an officemate. We don't ban you from dating people at the office, but we don't tell you it's a great idea, either.
If you get into bed with your cousin, there's no need for Uncle Sam to throw you in jail. If it works out, great. If not, you'll find yourself in a jail no uncle will let you out of. " http://slate.msn.com/?id=2064227
The Love That Dare Not Speak Its Surname What's wrong with marrying your cousin?
(and, the genetic issue would only involve barring reproduction, not sexual relations.)
"We're in a fucking stagmire."
--Little Carmine, 'The Sopranos'