Procreation

I'm sure everyone knows someone who really ought to keep their particular genes out of the gene pool. So why does it seem that those are the ones who seem the most easily able to procreate? And yet those that would have made excellent parents are denied. It makes little sense. I happen to know of 2 such people, the kind that probably oughtn't reproduce that is. The children are fine as far as genetics, quite aesthetically pleasing really. It's the parent-figure that is lacking. Or perhaps it is more that they are lacking in overall parental instincts. Whatever it is, it is one of the truly baffling phenomenon in our known existence to my way of thinking! So while I understand that it is the most basic drive of all species, I really wonder if some humans shouldn't more carefully consider before they choose to join the ranks of the generations.