Marriage between two first cousins is:

(a) gross.
(b) unremarkable.
(c) desirable.

How you finish this sentence has a great deal to do with where you grow up.  According to this New York Times article, most Americans find cousin marriage to be disturbing and testimonials from married cousins about their deteriorated family relations and social stigma attest to it.

In fact, cousin marriage is illegal in 25 states and several others have limits on cousin marriage.

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Emerging evidence that cousin marriages do not significantly increase the risk of birth defects suggest that the stigma and laws against cousin marriage are unwarranted.  A doctor cited in the study suggests that disallowing their marriages or rights to have children are tantamount to “eugenics or forced sterilization.”  Even if there were significantly increased risks of genetic disorders, Dr. Bittles argues, “People with severe disorders like Huntington’s disease, who have a 50 percent chance of passing it on to their offspring, are not barred from marrying because of the risk of genetic defects… so cousins should not be, either.”

In any case, U.S. aversion to cousin marriage is culturally and historically contingent.  That is, it is related to our particular time and place.  Worldwide, more than 10 percent of marriages occur between first cousins and cousin marriage, historically, has been quite desirable.