* Well-rounded in the context of a specific sub-culture, and such

The flip side being, a father is not an absolute necessity, so long as there are adults with testosterone-correlated traits (actual; not stereotypical) in the life of the child.

This could be one of the mothers, or it could be male figures in other areas.

It appears that having two parents present is desirable, this presents two possible models of adulthood for the child to extrapolate from and of course it means both parents are less exhausted from caring for the child as they were sharing the work (to a greater or lesser extent). It seems unlikely that it's more crucial that the parents are of different genders than that, say, they have different temperaments, different academic backgrounds, or a dozen other variations.

There are plenty of one parent families from which we can conclude that this is viable but not desirable, and not that many 3+ parent families from which we could decide whether e.g. seven is the ideal number of parents, all else being equal.