It sounds like you’re thinking polyamory is a requirement.
In any world derived from our own, polyamory would be allowed, not required.
If you’re worried you’d be judged harshly for raising a child with only two parents, as many on this thread are judging single moms... well, you’re probably right. But maybe that’s an argument for a little more support for everyone, not for less support for the more-than-two-grown-ups-per-household folks?