Why should society reward rapists by allowing them to forcibly breed? The law typically does not allow one to benefit from his wrongful actions.
Why should a raped woman, or a raped man, be forced into slavery or involuntary servitude, in violation of the 13th Amendment, by being required, in the former case, to carry the rapist's spawn to term and, in the latter case, to pay the rapist to support her spawn?
I don't think killing the baby is going to stop a rapist from raping. They do it out of uncontrolled hate, not a desire to have a baby. Having and giving the child to be adopted isn't forcing anyone into slavery...except that the woman has been forced to endure a horrible rape. But to further rape her by abortion is only going to make the emotional trauma worse.