What does a herd of politicians running to jump on the bandwagon sound like? The thunder of the hooves of panicked cattle? Or the rustle of lemming feet over a cliff? Choose your own metaphor, but the stampede of many US senators into the same-sex marriage corral just since the beginning of the year is remarkable.

The about-face by Ohio Republican Rob Portman was the most widely publicised, but there have been at least 14 others: Mark Kirk, of Illinois; Tom Carper, of Delaware; Bob Casey, of Pennsylvania; Bill Nelson, of Florida; Brian Schatz, of Hawaii; Claire McCaskil, of Missouri; Mark Begich, of Alaska; Jay Rockefeller, of West Virginia; Tim Kaine and Mark Warner, of Virginia; Jon Tester, of Montana; Debbie Stabenow, of Michigan, and Kay Hagan, of North Carolina.

Those are only this year’s lemmings. Last year, in addition to a number of senators, the Vice-President and the President announced that their thinking had evolved on the issue and that they now felt relaxed and comfortable with marriage equality.

The suddenness of the shift is certainly surprising, but in retrospect, it is easily explained. American marriage is like a sea cliff which has been gradually eroded at the base by the pounding waves. It may look firm from the top, but one fine day, the neighbourhood is bound to topple into the ocean.

Three undermining forces have been at work.

First, contraception. Since the beginning of the 20th century in the US, there was a growing tolerance of contraception. By the mid-60s, the contraceptive pill had become widespread. This was a tectonic shift in marriage culture. Procreation became a mere lifestyle option for a married couple. They could limit their family size to two, one or none – whatever fit their budget or lifestyle.

Second, divorce. Until the late 60s, divorce was relatively uncommon. But after no-fault divorce was legalised, divorce became epidemic. Four or five marriages out of every ten would fail. If couples no longer had to stay together for the sake of the children — because there were no children, romantic attachment became the only glue keeping couples together.

Third, increasing promiscuity. With the pill came the sexual revolution;  free love was free of consequences. Adultery was still frowned upon, but in most jurisdictions, it was decriminalised. “Sex is no more a moral issue than eating a good meal,” wrote a prominent sociologist who has taught at the London School of Economics, Catherine Hakim, recently. “The fact that we eat most meals at home with spouses and partners does not preclude eating out in restaurants to sample different cuisines and ambiences, with friends or colleagues.” While this may sound radical, it expresses a common attitude.

So, in summary, the last half-century has seen the erosion of the three cornerstones of marriage as traditionally defined: procreation, permanence and fidelity. In the minds of many, a new understanding of marriage has taken its place, marriage lite. This is marriage without children, marriage without commitment, and marriage without exclusivity. And these are precisely the key features of same-sex marriage.

The recent converts argue that they don’t want to change marriage. They only want to extend the boundaries. As Republican Rob Portman says, “ One way to look at it is that gay couples’ desire to marry doesn’t amount to a threat but rather a tribute to marriage, and a potential source of renewed strength for the institution.”

But this is nonsense. Gay marriage will strengthen marriage lite, not traditional marriage. It would never have reached lift-off velocity if adultery were a crime, or if the clock were wound back on no fault divorce. As a cynical Australian politician once put it, “Marriage long ago stopped being to the exclusion of all others and for life. If we don’t care about those two elements being disregarded by so many, why should we care about the ‘between a man and a woman’ part?”

These recent converts have not only accepted but defended the right to divorce, contraception and extramarital sex. So, in a sense, it is intellectually dishonest for them to oppose gay marriage. They sold the pass long ago on traditional marriage. The only surprise is that they took so long.

Michael Cook is editor of MercatorNet. 

Michael Cook

Michael Cook is the editor of MercatorNet. He lives in Sydney, Australia.