Gay marriage has come to be widely accepted because our society stopped thinking of marriage as a conjugal union decades ago.
Between five and six decades ago, to be precise. That's when the birth control pill — first made available to consumers for the treatment of menstrual disorders in 1957 and approved by the FDA for contraceptive use three years later — began to transform sexual relationships, and hence marriage, in the United States. Once pregnancy was decoupled from intercourse, pre-marital sex became far more common, which removed one powerful incentive to marry young (or marry at all).
It likewise became far more common for newlyweds to give themselves an extended childless honeymoon (with some couples choosing never to have kids). In all of these ways, and many more, the widespread availability of contraception transformed marriage from a conjugal union into a relationship based to a considerable degree on the emotional and sexual fulfillment of its members — with childrearing often, though not always, a part of the equation.This is what makes our job so difficult. The fatal premise was accepted, the poison pill was swallowed, long ago. We really do have to go all the way back to the beginning to explain why and how contraceptives got the ball rolling with the unravelling of the moral fabric of society. And when the overwhelming majority approves wholeheartedly of contraception, well, it becomes a monumental task.