I can acknowledge that rape and incest are things that have always happened. That's not where he goes off the rails. He goes off the rails by using it as a justification for not allowing women to abort the children who come from those circumstances.
First, he ignores that women have been aborting those children or outright killing them for all of humanity. Rape, incest, abortion - all part of the fabric of humanity that he's extolling. He cannot say 'Rape and incest are normal but abortion isn't."
Second, if his reason for not allowing women to abort the products of rape and incestuous unions because rape and incest are a necessary part of humanity then there is no argument for criminalizing the rape and incest. If the argument is that we don't want people committing rape and incest, how then can we argue that women who have been the victim of rape and incest should be forced to abide by the actions of the criminal?