I have a question about abortion?

Why do some pro-lifers say that abortion is okay if the baby was the product of rape, but not for another reason. If you think that abortion is wrong because it is killing an innocent person, what does it matter what the FATHER did? It makes it seem as if they are just bitching because they want to punish the mother for not living a christian lifestyle. Or they at least have some alterior motive for contradicting themselves like this.