morals would be taught by anyone raising a kid. you cant define it good or bad just by WHO it happened to be. it changes with every person, and there is no best.
be it the birth parents, grandparents, the kid's lesbian second mother, the television, the neighbors dog, or a rock the kid found outside.
by your logic, you are saying 2 married parents have been consistently perfect at raising kids for all of mankind's history. which we know isn't the case.