With the advent of reliable birth control, it became possible to have sex with a minimal chance of unintended pregnancy. Suddenly, the groundwork was laid for a sexual revolution in the 1960's, when a new generation discovered that the traditional reasons to avoid premarital sex no longer existed. The concept of withholding sex until marriage was no longer as vital as it had been.
As feminism increased equality, allowing women to enter lucrative careers where they could support themselves, the need to marry for economic stability almost completely left the picture. No longer would a woman need to preserve her "virtue" to trade for economic support, as she could now support herself.
This progression opened up a vast variety of lifestyle options. With sex no longer leading to parenthood, people could have satisfying sexual relationships without them needing to provide economic support necessary to take care of children. The stability of a monogamous relationship, traditionally needed to support the family unit, is no longer necessary for many people who do not want children.
Interestingly, this same social progression has led many people to believe that marriage is no longer even necessary for raising children. In my work, meeting with parents, I find that less than 20% of the parents I meet are married to the parent of their child. Single parenting is no longer shameful, as it was in previous generations. In fact, it is not even seen as irregular.
As marriage ceases to be the norm, some people feel less social obligation to take care of spouses and remain committed their relationships. As many act less trust-worthy, many choose not to trust. Expecting that they will eventually be abandoned, many people chose to develop their own independence, financial and others.
The traditional American family is no longer. The future which conservatives fear is here, and it happened long before gay marriage was legalized. Yet, even with all these fundamental social changes, society goes on, the farmers still farm, the sun still rises and sets, and chaos does not reign. We live in uncharted territory, and most of us have lived in for our entire lives.