I'm speaking as a matter of Catholic morality, not the child outcome standards that you speak of.  If child outcomes are our barometer for morality, let's take all the kids away from poor people and give them to rich people, and stop the rich people from having their own kids.

For you worldly types, psychology will tell you that the loss of a parent is a devastating experience for a child; part of that is the loss of what one had, but part of that is the loss of what one stood to have going forward.  You start with one parent, you've already suffered the devastating loss of the other.