sterilization not working?

I am doing a playthrough without mods and I have some boomalopes tamed for the chemfuel they provide, I started with 2 females and 1 male. I put a sterilization task on the male but before it got done, he had already done his job and both females were pregnant. No problem, he is sterilized now so that will be the last generation. The kids were 2 females and 1 male combined so I sterilize the male while he is still a child but now the 2 oldest and one of the babies (already grown up now) are pregnant.

Am I overlooking something or is this a bug?