Except that this is demonstrably already not true.

As research psychologist Peggy Drexler notes: There is a perception in the United States that most American children grow up in a home with a married mom and dad, but really, less than 23 percent of American households fall into that category. There are an estimated eight million women parenting alone, and at the very least another 100,000 families with two gay moms.

Lots of poly people parent effectively and well. Depending on the particular arrangements, some kids of poly folk benefit from being part of a larger clan sort of arrangement. Loneliness is a terrible epidemic in America. Having warm relationships with all sorts of other "as family" is a good thing for many children.