I really struggle why we keep maintaining some archaic definitions around the “family unit” anyway. So someone has 3 parents instead of 2 - nothing wrong with that.
It's not that hard to imagine a rational argument where humans have evolved to grow with one or two parents, leading to all sorts of psychological prewiring.
(I'm not supporting the argument, just saying that it's not hard to come up with a plausible rational one)
To what extent? for example, imagine they still had identifiable parents with some some prominent role. I could also see the argument that parental role is diminished in modern society with the outsourcing to public schools.