I'm curious why you and OP are equating "not monogamy" with "single-parent?" What about polygamy? Or even non-married but still-committed group partnerships?

I'm certainly not suggesting to eradicate monogamy and replace it with these, and it wouldn't make sense to say any one of these is holistically better than the others, but if we're focusing on just the "what's best for kids" angle... wouldn't it be better if kids had _more_ parents/very close adult relatives? More people in their life they can rely on for attention, advice, care, etc.?