Actually, natural law isn't a proper "element"...of the Magesterium, it's instead something that existed before the Magisterium, and thus is a source drawn upon and considered by the Magisterium in weighing new issues. 

As we get more and more evidence for instance of the beautiful complementarity between a man's body and woman's body...Chromosomes, cells, even the marvelous welcoming behavior that a woman's immunological system provides for "foreign cell matter" (sperm), which would under other conditions be attacked immediately by her antibodies....this new natural knowledge gives the Magisterium something to reflect on.

It deepens the Church's teaching on the unitive and procreative purposes of marital intimacy between a husband and a wife.