again, it's by orgasm. your keeper (which you've said is the futa,) is the only one affected by the "cum" command. in my experience it takes a lot more work and time to get the keeper's partner to orgasm, allowing the keeper to get pregnant.
whups, that was a mistake on my part, i forgot "not."
only the futa is getting pregnant because only the female is cumming. for the female to get pregnant, the futa must orgasm.
because the base chance for pregnancy is only 35% (without the fertile trait,) it generally takes a few attempts to get the keeper pregnant. because only the keeper can orgasm on command, it can take a while to get a successful pregnancy.