Regardless of what is done to the rapist, some girls and women may just not be psychologically able to allow their bellies to get big and ever bigger with the result of an attack. It's one thing to leave someone alone even though they remind of an attack and quite another to be carrying that incessant reminder inside your body, your belly advertising it to the world, and hear "Congratulations!" about a horrible crime committed against you.
Of course, I'd prefer to prevent rape or to have more girls and women taking effective contraceptives. But we live in a very imperfect world.
It's hard to imagine many women being disappointed when they've been raped and the period is on time. "Drats! I so wanted to have the monster's baby!"