We have more than satisfied the command to populate the Earth.

The celiebates who make up the policies of the institutional Church have decided that the ONLY proper reason for having sex is procreation. Augustine, who was actually a moderate on this topic, said that any married couple who had sexual intercourse without the specific intention of procreation was sinning. 

In the midst of this, there are sane people who say "The Catholic magisterium is talking through its hat when it comes to sex." I would consider Marty to belong to the phallus cult.