Got online to look this up this morning. Good advice for long-marrieds from the Duchess of Devonshire:
She was famously lovely; her husband was famously dashing. He inherited the dukedom only because his older brother, Billy, was killed in World War II. It was a time of constant loss. “Two of my brothers-in-law,” she said. “My only brother; Andrew’s only brother; my four best friends—all killed within a month of each other.” But her generation believes in self-pity about as much as it believes in self-esteem. “What can you do?” she asked. “Blow after blow came, but there was absolutely no reply, was there?”
Her marriage lasted 62 years, surviving Andrew’s long bout with alcoholism, as well as his discreet dalliances. “It was absolutely fixed that we shouldn’t divorce or get rid of each other in any way,” the duchess said. “It’s completely different to Americans, who all divorce each other the whole time. Such a bore for everyone, having to say who’s going to have the dogs, who’s going to have the photograph books.”
Encouraging people not to regard divorce as the only possible response to an infidelity would go a long way toward strengthening the institution of marriage. It would do more to undermine our "culture of divorce" than preventing same-sex couples from marrying ever could.