The argument isn't that having been raped is a sin, the argument is that it's an unfortunate eventuality which shouldn't be blamed on or taken out on the baby - i.e. the baby shouldn't be killed for it.

The irony of these people arguing that a rape victim should endure a parasite in her body for nine months to save its life, but the same people finding taxation to save the lives of the poor an intolerable burden, appears to be entirely lost on them, alas.