Don't care; people can do what they like. The risk of genetic defect if there are children is less than one may expect; the effect of the parents being cousins is, if I recall correctly, similar to the effect of the mother being relatively old, and nobody says that 40 year old women having children is disgusting, for instance. If all you have against it is an intuitive discomfort with the notion, unsupported by any reasoned argument, then you really ought to reexamine your principles.