Deleterious in what respect? If there is a 'stay at home mom' and two working parents, seems like the family might be more secure. It's like Heinlein's ideas about the evolution of family structures are starting to emerge. There are biological considerations in pair-bonding, but how much of that is learned behaviour?
The same argument could be (and probably was) made in the case of children conceived and born within the "Heather Has Two Mommies" construct. Didn't seem to help. Further, a case could be made that children born into a polygamous family could even do better at least financially than a single mother/father family. For example, the father and one mother works while the other mother stays home and takes care of the children.