If its consensual sex between adults then who are we to criminalize it?
It legalizes incest, clear and simple, no matter how you choose to word it or to skirt around it.
No civilized society with which I am familiar legalizes incest.
While illegal, its not enforced most times --if it was consensual and between adults. I'm not advocating in favor of it, but we can promote "not incest" (so to say) without having a law prohibiting it. Similar to drugs being legal but we don't promote drug use.
On top of that is the question of how many degrees of separation is incest? Is it immediate family? First cousins? Second cousins?