'Responsible Parenthood'  is a teaching of Paul 6 and  Humanae Vitae that is under-appreciated.

Paul 6 had the wisdom to appreciate that God requires the virtue of Prudence within families. And God gives the right and responsibility to parents to make decisions and choices concerning pregnancy and births.

True, he felt obligated to follow the opinions of his ancestors, but the principles he laid down give the foundation for going beyond them, as many people saw and judged for themselves.

He did have the wisdom to see that it is the very 'natural law' for human sexuality that pregnancy could occur – and that a woman is 'open to conception' - only during a short period of the monthly menstrual cycle.. And that it was permitted, as had already been established by the CDF, to engage in sexual intercourse when the 'procreative function' of sex was not possible.