A question for Catholics

So this discussion of whether Catholic hospitals should be required to provide emergency contraception to rape victims is basically a giant trainwreck, but I am actually curious about what the theological objection is here. Plan B, the emergency contraceptive at issue, prevents ovulation and has not been shown to interfere with the implantation of a fertilized egg.

I am under the impression that the Catholic objection to contraception centered around the view that sex is appropriate only in the context of a marital relationship and should combine both love and openness to procreation. How does this work in a rape scenario? What objective is fulfilled by keeping a rape victim open to the possibility of procreation in the wake of a nonconsensual sex act?

I am not trying to be snarky. I actually want to know how the doctrine justifies this.