Epic fail

Playing devils advocate (maybe literally) the idea that Incest is an abomination of sorts surely only old cultural hang ups or religious dogma?

If contraception could be relied on, maybe. Children born from incest are at huge risk of getting two copies of damaged recessive genes, so it's irresponsible to risk conception of such children.

This is kind of a bug in the way natural selection works. As bad recessive genes become rare, individuals who have two copies of them and so express the gene become rare squared, and the selection pressure against the damaged gene dwindles. We end up all carrying around damaged genes, but generally all have different selections, so get working versions from our other parent. But breed with someone too closely related there is far more chance that the offspring will get two bad copies of something.