Marcus, you raise an intertesting question. I'd never assumed when reading these fics that Ultrawoman's motivation was as you describe. I always got the feeling that the explanation that Ultrwoman was the mom was pretty offhand. The reader could make of her disappearnce what they would, and given that Lois was, in fact, Ultrawoman, that was that.
So here's my thought. It's Ultrawoman's narrative, rather than Superman's.
Ultrawoman is an intergalactic traveller, backpackinging around the universe so to speak. She likes Superman, decides he's good genetic material and decides to have a child by him. She's decided to have 3 or 4 kids, but wants to ensure the survival of her genetic material. So, following the advice of investment counsellors everywhere, she diversifies, wisely not wanting to put all her eggs in one basket.
In her culture that's how it goes.
When the kids reach their mid-teens, they are sent to reside with their fathers for a bit - time to learn more stuff.
c.
