Precisely! And more than that, even if propensity to rape is both heritable and highly variable in the population (I've no idea if this is true), it doesn't follow that by culling some children of rapes the total number of rapists will go down.

Rapists can have children in normal marriages, like everyone else. And I've read somewhere that most rapists commit more than one rape; if true, then the mandatory abortion policy must be enforced in almost 100% of all rapes to drive the numbers of rapists below replacement rates.

Also, since the postulated rapist genes haven't been selected out of existence, there is probably some balance of rapist vs. none-rapist genes in the population. If we cull some but not all rapists, then we might create incentives for the remaining rapists to breed more to make up. And we'll be selecting for more successful rapists, resistant to our methods of catching or of punishing them.