First of all, the trait is obviously recessive. Otherwise, Keanu would have 1/4 to 1/2 a chance of having an Anti-Christ baby regardless of who he impregnates. The only reason Satan would HAVE to pair up his half-children would be if the trait was in fact recessive, requiring both parents to at least be carriers. Additionally, if the parents are mere carriers of a recessive gene, they wouldn't exhibit any of its characteristics, which is why Keanu's character is such a wuss in the film. So, even with two carriers, there's only a 1/4 chance of having an Anti-Christ baby. Not good odds, but if you're Satan, you've got all the time in the world.

Second, having multiple babies increases your chances overall of having an Anti-Christ baby, so Satan's idea is a good one even despite the recessive nature of the gene. Satan's chances of having an Anti-Christ baby are represented by the equation 1 - (1 - 1/4) ^ n, where n is the number of grandkids Satan has. After just 3 grandkids, Satan is already has a 57.8% chance, which ain't too shabby. And since this function is exponential, by around 8 kids or so, Satan has a near 90% chance. Thems's good odds.

Third, I want Wawa as well, and I hate you.

--Jeff