You may recall one of my Pithy Pet Phrases is that "Christianity is the ultimate pro-choice position." It well complements the one in my sig, but also makes this issue (most issues, really) hard to separate from theodicy. Choice makes all the difference in the world between a desired and undesired child. To say God enabled the first and allowed the second is not to say both are Gods will.

Where pregnancy is not practically impossible (as in my mothers case,) I contend Gods role limited to having made sexual reproduction possible. That does not make all the forms in which we accomplish it Gods will any more than giving us brains makes the atom bomb Gods will. Where pregnancy is almost or wholly miraculous for those who want children, once again God enabling a choice is not the same as God allowing what no one chose. If Mr. Mourdock knows of a woman who:

1) Medical science declared incapable of conception, 2) did not want a child in the first place and 3) nonetheless got pregnant from rape,

THEN he might have a case on the grounds you cite, but only then.