When sex loses its power to transmit life, it becomes just a fun and casual pasttime. Inhibitions and sanctions against sexual adventurism break down. The illusion of freedom from weighty ramifications lead more and more people to indulge in promiscuity, adultery, etc. When sex loses its power to transmit life, the unique rights and responsibilities of the married state are forgotten. Divorce becomes common, and confusion grows as to what marriage is all about.
If marriage is not about transmitting life, if it is just a contract between two adults, then who's to say those two adults must be of opposite sex? Who's to say it must be limited to two? Who's to say it must be limited to adults? Who's to say it must be limited to humans? What is thought ridiculous today may be seriously considered and adopted at some future point, if marriage is not about the transmission of life.
Contraception is the root of abortion. It appears to be the root of much else that afflicts our troubled culture.