Groucho and Dan, you have touched upon an interesting conundrum. At least one legal scholar has said in recent years that it makes little legal sense to require de facto licensing for adoptive parents but not for parents having children the old-fashioned way, especially in the wake of so many examples of abuse, neglect, and just plain poor parenting as with Louie's post. That comment came after an academic, Jack Westman, proposed licensing all parents in a book that came out in 1994. He estimated the cost to society for each abused and neglected child at as much as $3 million because of wasted potential, unproductivity, and crime over that child's lifetime.
Westman, in turn, had proposed that licensing most parents should be as simple as getting a driver's license provided that (1) the parents were at least 21, (2) the parents were married to each other, (3) the parents could show that they were self-supporting, (4) neither had any felony convictions, and (5) both had taken a parenting class, which could be given in high school. In other words, for those who met those criteria getting licensed would be easy. They would sign a contract agreeing not to abuse and neglect the child, and violation would therefore constitute breach of contract with civil penalties.
Others would have to take various steps to prove fitness to keep children after having them, and obviously the whole proposal opens huge cans of worms that I won't get into, which would quickly make this thread political. But I see no real problem with Westman's proposal to license natural parents using those criteria and maybe adding a requirement that both parents be able to pass random drug testing during pregnancy and while the child is an infant.
I've seen far too many children put through the wringer because of bad natural parents. And just a closing thought: everyone today rails against the idea of child labor as was common 100 years ago, with children as young as 6 working in hazardous factories for 12 or more hours a day. What everyone forgets is this: were the companies operating the factories just grabbing the children off the street and making them work? No. The children's parents were sending them to the factories to earn money that the parents then spent. Just to support the family would be bad enough, but in parts of the country, certain parents were notorious for putting their kids to work to support daddy's drinking or opium habit. Children are not always best off with their natural parents.
I also knew of a number of older people whose parents forced them (long after the bad old days of really young children doing child labor) to drop out of school after the 6th or 8th grades to go to work for money for the parents, thereby shortchanging these children's future as adults as society changed over the years. Typically, this would be to work on the family farm or some such. Yeah, I know, we're judging events of the past by today's standards, but even then, many people found these practices questionable or repulsive. And there isn't a lot of difference between these examples and your average crack mom of today forcing her young children into turning tricks to support mommy's drug habit. It's a difference of degree, not of kind.