Nice topic Lalitha, and if I may say another controversial one. Don't think I will be able to give much time on this thread, though, seeing that I am extremely busy on the other thread.
I haven't come across any solid scientific theory that classes incest as a natural tendency (contrary to homosexuality, which is classed as a natural tendency by almost all authentic psychological departments). Correct me if I'm being wrong in any way, aren't kids born out of siblings or very close relatives more prone to be infected with diseases? I'm not quite sure, and since I haven't done much studies on this subject (as this doesn't seem to be as controversial and as a throbbing issue like homosexuality, or abortion is), I can't say it for sure, but as much as I have come across, I heard that child[ren] born put of very close relatives have biological problems; here is one short link from Wikipedia on that matter. So if they are a heterosexual couple, they cannot have kids, and always have protected sex.
Also there are parents who sexually abuses their child, and that must be prohibited.
While incest might also be a natural tendency, and something a person is born with (i.e. not a choice), it must be a minority who are born like that, or grow up like that.
Personally, I am on a fence for supporting incest. While I believe in the "Live and Let live" and "Only God will judge me" argument, I cannot support incest if it involves people of opposite sex (or at least I cannot support them having babies).
Let's see how others react to it. I don't think this topic is going to end up nicely. DOn't know if it is applicable for IF either, 'cause we have 13 years kids, too.