[Nerull]

Um...I think he meant that they were not actually "sleeping" (i.e. doing "other activities" instead).

You are right about that other thread, Illumina. I remember it too, but I'm too lazy right now to search for it. But the general idea is that he changed his race to match the mother's race, so the child came out the same race as the mother. In each case, he had a completely different form, so a completely different genetic makeup. He just infused each with a portion of his power. Thus, the other Bhaalspawn are technically your half-siblings due to the same father, but genetically there is no common ground.