Actually, if you use a punnet square (if it's the thing I'm thinking of), there is only a 25% chance of passing on the recessive trait, assuming both parties are carriers. That may be too high of a risk for some people, and obviously genetics is a lot more complicated than a square, but still, it's not as bad as people like to make it out to be if it's just a one-generation thing. Now, if you have several generations worth of incest, then, yes, there is more of a risk for genetic defects.
The Egyptians and European Nobility went a little nuts because it was nothing but incest. The Egyptians believed the pharaohs were living gods, and therefore, it was blasphemy to dilute their blood. Each pharaoh married his sisters, female cousins, and even mother and aunts if they were still alive and could bear children. His queen had to be a sister, and she was supposed to be the mother of his heir. It was pretty much the same with European Nobility. They were too good to dirty their blue blood with common blood, so all they had was each other. Over time the gene pool got smaller and smaller.
But we're not talking about several generations here. Other than admittedly finding one of my first cousins kinda hot, I don't really understand the attraction. However, I don't understand the attraction to furries either. That doesn't mean there is anything wrong with someone being turned on by furries, it just isn't for me. I support the right of consenting adults to have any kind of relationship they wish, be it homosexual, incest, or polygamy. There is the possibility of abuse and coercion, but you have that in any dynamic. Police officers, teachers, and bosses have already been mentioned, and there are probably other examples I just can't think of right now. Just because the possibility exists doesn't mean that all such relationships involve abuse and/or coercion.
Okay, so here is food for thought. An older sibling has some influence over a younger sibling. That is usually true. Now, think back to high school. You, a lowly Freshmen/ Sophomore see this really cool Senior. You want that cool person to like you. For some reason, said cool person notices your existence. Again, you want them to like you, and you want to please them. This happens, more often with older boys and younger girls. The girl is so psyched that the hot older guy wants them that they end up doing things they wouldn't normally do; like sex. For argument's sake, let's say the guy didn't do anything like say he'd break up with her if she didn't sleep with him. He just made it clear that's what he wanted, but would've taken no for answer. Is that coercion? Should that be criminalized, even if he did threaten to break up with her?