The difference is the abnormality is the same in the child as the adult had. A better, but still flawed analogy would be if we were to outlaw people who each carry the genes for
Tay-Sachs from reproducing. A couple of differences there, the child is born live, and generally healthy, just terminally ill. Also, in THAT case the result is inevitable. As I said though Multi-generational incest is the big concern, single generation incest IS low risk, but it's much easier to outlaw incest than to verify your parents weren't an incestuous couple as WELL as you. And the risks to children in that case are ENTIRELY the creation of the parents and preventable. So society should.
Incidentally, if you're curious - my position is parents who carry the genes for tay-sachs, if they get married should not risk having children. It's a horrible disease. But it shouldn't be illegal either.
I don't think you can legislate it that way under an equal protection argument as you suggest. Incest should remain illegal in all it's forms even when having children is impossible.