As I've said on another thread of this type, I'm adopted, and am unable to easily (IE: without a whole bunch of time and legal fees unless I need a transplant right now or some-such) get info on my birth parents. Ironically, about the only folks I know for certain aren't related to me are the folks in my adoptive family... (Thus my fondness for short women with dark hair. I'm tall, adopted, and have a very strong desire to never find myself on the Jerry Springer Show...)
All that said, while it's genetically no big deal for close cousins to hook up for a generation every now and again, it is a big deal socially. As it should be. It keeps this from becoming a regular event, which would be a Bad Thing from a genetic standpoint. (cf: Dunwich & Innsmouth, MA, various royal families, and the extras in "Deliverance") One of the few cases where shame is still allowed to provide a socially useful function, but that's a rant for the politics area. I view close cousins schtupping much the same as I view polyamorus households or, to a lesser extent, homosexuality. If it floats the boat of everyone involved and everyone involved is of legal age, you'll get no more than a raised eyebrow from me. (Though, I'm more accepting of homosexuality than I am of the other two as it's more self-limiting as we currently understand it. Again, a seperate topic.) However, it should happen on the QT and the folks involved should expect to be socially rejected because that sort of lifestyle has very negative effects if widely adopted. Genetic issues with the family marriages, young men with no hopes of marriage in polygamous societies, unintentional incest with group marriages or "Free Love" set-ups, reduced offspring with widescale adoptation of homosexuality. (Again, that last is unlikely as it seems to have a genetic cause for folks who're truly "gay", however you'd be amazed at what folks will do to follow a fad...)
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"If it ain't the Devil's music, you ain't doin' it right!" -Chris Thomas King