When they first met on a beach in Fort Lauderdale, former Senator Harris Wofford was 75 and his soon-to-be husband, interior designer Matthew Charlton, was just 25.

Now, 15 years later, the Washington couple plan to tie the knot on April 30, exactly three weeks after Wofford's 90th birthday and 68 years after his first wedding, to Clare Wofford, to whom the former politician was happily married for 48 years before her death.

If all that maths is more than confusing, Harris Wofford recently provided a simple common denominator: "It is based on love," he wrote in the New York Times on Sunday.

In a moving (and surprising) personal essay that spanned his meeting Clare when the two were student activists during World War II and then Charlton as the former senator from Pennsylvania was settling into life as a widower five decades later, Wofford directly challenged the need for labels.

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"To some, our bond is entirely natural, to others it comes as a strange surprise, but most soon see the strength of our feelings and our devotion to each other. We have now been together for 15 years.

"Too often, our society seeks to label people by pinning them on the wall - straight, gay or in between," he wrote. "I don't categorise myself based on the gender of those I love. I had a half-century of marriage with a wonderful woman, and now am lucky for a second time to have found happiness.

"For a long time, I did not suspect that idea and fate might meet in my lifetime to produce same-sex marriage equality. My focus was on other issues facing our nation, especially advancing national service for all. Seeking to change something as deeply ingrained in law and public opinion as the definition of marriage seemed impossible.

"I was wrong, and should not have been so pessimistic."

A friend of Martin Luther King Jr's who served as John F. Kennedy's civil rights adviser, Wofford joined with Rep. John Lewis, D-Ga., during his single term in office to co-author the King Holiday and Service Act, legislation that would marry the national holiday with volunteerism.

In his essay, Wofford wrote that he once believed marriage equality to be impossible but eventually had a change of heart, using his past experience as a guide for the future.

"I had seen firsthand - working and walking with the Reverend Dr Martin Luther King Jr - that when the time was right, major change for civil rights came to pass in a single creative decade. It is right to expand our conception of marriage to include all Americans who love each other."

There are no details on the April 30 ceremony, but according to the essay, Wofford and Charlton plan to recite traditional marriage vows while joining hands and promising "to be bound together".