Marriage vows say “I will love you forever.” Historically that meant 10-20 years because humans died younger. In fact, in no society on the planet, until the early 1800s did more than 2-3 percent of the population live to the age of 65 or greater.

Things change. US Census Bureau reports show that 18 percent of marriages occurring in 1910 reached 50th anniversaries. Of marriages taking place in 1930, 26 percent lasted 50 years or longer.

But, by the 1990s, divorce had become so commonplace in America that only three marriages in 100 last 50 years. What happened?

When famed anthropologist Margaret Mead was asked by an interviewer to comment on her three “failed” marriages, she shot back that NONE of the three were failures. “Each was a very successful, satisfying union.”

Thanks to her long life, Mead had gone through at least three distinct adult stages. Each stage had different needs and priorities, and in each stage she found a mate who satisfied them quite nicely.

Sociologists call this “serial monogamy” and it may be the route for marriages in today’s historic “avalanche of aging.” Americans life-expectancy is 78 today. American women aged 50+ who have not had a heart or cancer encounter, can expect to live to 92. Thus, life for today’s second-halfers may be too big for one mate.

However, with the kids grown, many happily married couples report a new “golden age” of love, sexual delight, and happiness. Even before Viagra or Cialis, couples in their 50s, 60s, and 70s report the best love-making of their marriages. However, if marriages have not been healthy at their core, they quickly crumble once the children are grown.

Fifty years ago, loveless empty nester couples in their 50s or 60s would stay married because they knew that one or the other of them would die within a few years. Today, being newly single at 60 means two to three more decades of living — too long not explore romantic opportunities.

Given the numerous challenges of more single second-half women than men (currently the ratio tops three women to two men at age 75), we are seeing instances in which more than one woman shares a man (happily) with other women. One single Eastside male friend of mine has four or five close single female friends to join him for the Rep, tennis weekends or Sunday dinners. The women like the arrangement because they value their personal freedom, and they enjoy my good friend — but not on a 24/7 basis. Like all the best things in life, moderation works perfectly.

Bill Morton has a Certificate in Gerontoly from the UW and is the author of “2H: The Official Second Half Handbook.” He’s lived on the Eastside for 20 years.