Quote Originally Posted by Oneironaut View Post
Most of it (IMHO) is just society trying to enforce a status quo. Fundamentally, I'd say it goes back to the whole 'genetic disadvantage' thing, but in order to control something like that, you have to make the stigma go much deeper than that. It's not good enough to just let people think that "well, it's ok, as long as we don't have children." You have to demonize the act, in all its forms, so that people will stay as far away from it as possible. It's to a point, now, where you couldn't even mention having a relationship with your cousin without risking social ostricism(sp?).

It's a dogmatic insurance policy, so that the extreme case (degeneration of the species, due to inbreeding) is much less likely to happen.

Of course, that's just my speculation on it.
It's also my speculation on it. However, I don't think it was consciously planned out that way by humans. I think the response evolved into our biology because of how much incest screws up offsprings. I guess I am agreeing with several people here.

I should admit that I think the idea of lesbian twins is pretty hot.