What is the argument here? Yes, all things being equal, most people would probably agree that a child is statistically better off in a family where the parents are married and in a stable home. That has been backed up by God knows how many studies over the years.
If there were enough qualified and suitable married couples that were willing to adopt out there to adopt every child that needs to be adopted then there would be no reason to open adoption up to single parents and non traditional families. In such a scenario, every orphan would be matched with a storybook family and we would all live happily ever after in a Utopian paradise on earth.
Thats not reality though. The fact is, there are far more orphans in this world at any given moment then there are qualified and suitable married couples that are willing to adopt. So its rather unconscionable to limit adoption just to them. We know from numerous studies as well that nontraditional families can and do raise perfectly happy, healthy, and well adjusted children, so their is no reason to exclude them.