Forbes points to economics:
Basically, marriage is a market, like steel scrap or copper. Small changes in the supply/demand balance can produce huge swings in prices. Between 1950 and 1970, the “price” of women’s sexual favors collapsed, while revisions in welfare rules made it possible for low-income women to effectively “marry the government.” At the same time, changes in the economy made it more and more practical for educated single women to support themselves and their children without the help of a man.
And Mark Oppenheimer at Square Space wonders why conservatives aren’t as concerned about divorce as they are about gay marriage:
So here’s my question to Douthat, Maggie Gallagher, Ross Douthat Brian Brown, the world of conservative evaneglical preachers, and others who are so concerned about same-sex marriage: What does it do your perception of Ronald Reagan that he was a divorce–and in being the first divorced president certainly helped remove any last shreds of stigma? Would you have voted against him for that reason–as many would have in 1952? Would you discourage people from listening to radio hosts who have divorces in their past (Limbaugh, Dennis Prager), or voting for divorces like John McCain? If our goal is to work our way back to 1950 Marriage, how are we going to re-stigmatize divorce for wealthy white people? How are we going to make their divorces seem unseemly? In 1950, when a divorced woman moved into the neighborhood, people whispered about her. Are we prepared to whisper again? Are we prepared to tell our own children, someday, to stay in unhappy marriages to avoid the stigma of divorce? If your daughter gets knocked up by some really unsavory guy with no job, are you prepared to tell her to marry the slug?
But to hear most conservatives tell it, marriage was in great shape until the gays came along and wanted to destroy it. They ought to get their own houses in order before throwing stones at folks who only want to join the party themselves.