I don't really care with whom anyone sleeps with, as long as it's two consenting adults. I do however think that people should not be allowed to birth any offspring from incest. By the way, I count first cousin as incest. Statistics and data speak for themselves on this matter however sensitive the topic might be for some from a cultural/religious perspective.
Playing devils advocate,,what if it was possible via gentic screening or adoption, other ways to get healy children?
I would then allow kids if it was possible.
Plus i susppect more relgious people allow insest due to religious grounds such as may christain cults in usa...south asias ect