Plus I believe the current scientific line is that a single case of incest doesn't generally cause problems in any resulting children; it's inbreeding repeatedly down several generations that causes genetic problems.
And remember that one episode of House with the married couple? He was white and she was mixed? She never knew her father and they grew up in the same town. Everyone said they had the same eyes. They ran away and got married because his father hated her and threatened him if he stayed with her. Turns out he was her father and they were half siblings. He knew it, but no one else did. Creepy.
But here's a question: Is marrying your 1st cousin really any different than marrying your sibling? Not that I'm saying siblings should get married, but, genetically, 1st cousins aren't too far off than siblings, especially if they're double 1st cousins (his mom and her mom are sisters, his dad and her dad are brothers), which happened a lot back in the day. I've done a lot of genealogical research and have found several 1st cousins getting married in my tree, and several double cousins getting married, although they were two different degrees depending on the parent.
Not that I agree with it, but if two consenting adults want to engage in incest, I don't see a problem with it. It was quite common in the royal houses of Europe. Of course, they also had a lot of problems. Like the big chins. (A ruling family of Rome; I don't remember which one) And they were/are all related.