I don't disagree, but I think that there are also common sense social reasons.
For example, God prohibited men from marrying a woman and her widowed mother, in a time when polygamy was still practiced. There's no genetic reason for that, but it's still incest of a sort. He understood the emotional devastation of such unions and the rage it could spawn.
When cousins are prohibited from marrying, and incest is taboo, it not only prevents intra-family dogfights, it also expands the pool of men with no sexual interest in a given woman and who would be motivated protect her from exploitation and rape.
