Certainly a couple, by themselves, is the 2nd worst choice for raising a child other than a single parent, by themselves. Children need to be raised by lots of caring adults. However monogamy does provide a sort of back-stop safety protection against adults prioritizing their own relationships with each-other over their relationships with their children (who are naturally less interesting and invigorating relationship partners). Non-monogamous raising of children probably has its own advantages and dangers, the point is to make sure that children are always protected and nurtured to the best of their caregivers' ability. I worry about diffusing responsibility for raising children too much, at the expense of them developing a secure attachment. However, as long as all the adults in a child's life are prioritizing that child developing a "secure" attachment style, I could give a shit who is fucking who.

And yes, this is very much my business, because how you raise your children dictates what kind of society my children will live in, so please raise them right.