Each April, as lilacs push their way out of the dead land, I provide my Brit Lit students an overview of the early 20th Century, showing them how art reflects a culture’s history, science, psychology, philosophy and how the alchemy of art can alter the way in which we see the world.
I show them these three photographs, taken a mere decade or so apart:

circa 1910

circa 1917

circa 1921
See what you get when Picasso and Planck start to play ping pong:

Picasso: The Guitar Player 1910
Obviously, between the years of 1905 and 1920, Europe underwent a radical change in perception, a cultural revolution, and you don’t have to don a deerstalker hat nor peer through a magnifying glass to identify the motive in this case. I offer the analogy of European culture as a gorgeous stained-glass window – a millennium in the making – that has been shattered by “the Death of God,” the de-objectification of science, and WWI. Modernists like Eliot and Joyce and Picasso pick up the pieces, arrange them in different patterns, trying to make sense of chaos.
Poi s’ascose nel foco che gli affina
Quando fiam ceu chelidon—O swallow swallow
Le Prince d’Aquitaine à la tour abolie
These fragments I have shored against my ruins
Why then Ile fit you. Hieronymo’s mad againe.
Datta. Dayadhvam. Damyata.
Shantih shantih shantih
TS Eliot, The Waste Land
To my mind, the enormous sea change in the public’s perception of homosexuality in the last ten years rivals that of the rapid cultural transformation embodied in that shift in fashion from the corsets and bustles of the Edwardians to the short flimsy dresses of the flappers.
However, in the case of homosexuality, the reasons for the shift in the public’s attitude are not that obvious. I’ve been soliciting opinions from my colleagues and friends, and they not only disagree, but occasionally offer opposing suppositions. Like virtually everything else, the phenomenon is probably much more complicated than it might seem.
Obviously, one possibility for the shift in the public’s perception of same sex marriage lies in an erosion of traditional Christianity, which you could certainly argue is the case in post-Christian Europe. However, with 77% of Americans identifying themselves Christians in 2009, and given that the movement within Christianity has been a shift from mainline to evangelical churches, it seems dubious to suggest a “falling away from God” is the reason for the increasing tolerance of “the love that dare not speak its name.”

Oscar Wilde: Who would ever have guessed?
In fact, presupposing that the temple/church has possessed a monolithic view of marriage through the ages doesn’t stand scrutiny – as the shift from the polygamy of the OT to the monogamy of Salt Lake City s attests. Actually, traditional European marriages were established on the bedrock of property rather than on misty marshes of eros[1]. In fact, from what I understand, this loving-your-spouse innovation arose from of all places, English Puritanism. John Milton himself wrote tracts espousing[2]incompatibility as grounds for divorce. Here’s a layperson-friendly summary from Stephanie Coontz[3]
Parents arranged their children’s unions to expand the family labor force, gain well-connected in-laws and seal business deals. Sometimes, to consolidate inheritances, parents prevented their younger children from marrying at all. For many people, marriage was an unavoidable duty. For others, it was a privilege, not a right. Often, servants, slaves and paupers were forbidden to wed.
But a little more than two centuries ago, people began to believe that they had a right to choose their partners on the basis of love rather than having their marriages arranged to suit the interests of parents or the state. Love, not money, became the main reason for getting married, and more liberal divorce laws logically followed. After all, people reasoned, if love is gone, why persist in the marriage? Divorce rates rose steadily from the 1850s through the 1950s, long before the surge that initially accompanied the broad entry of women into the workforce.

A slippery slope indeed: John Milton enabling George Jones
In keeping with history, friend/colleague posits that the AIDS epidemic is responsible for the sea change. The epidemic underscored that homosexuality was more widespread than heterosexuals imagined and acquiring the disease (unless you were a hemophiliac or a junkie) became a sort of outage in itself.
As more and more exited the closet, this theory suggests, the more we discovered that this once-called abomination was prevalent among kind, gentle souls, the saintly professor, your first cousin, your brother, your son or daughter (cf Dick Cheney).
This theory seems reasonable to me, but I would also add that, at least among the educated, it has become abundantly clear that sexual orientation is essentially innate. I certainly didn’t make the post-pubescent decision to become hethero/(morose?), so it doesn’t seem likely that others would decide they would choose to be gay. Why would someone, especially in the ‘50s and ‘60s when “rolling queers” ranked just behind quail hunting as a redneck pastime? And, if indeed, you’re born gay, shouldn’t you be allowed to fall in love and to marry?
Whatever the reasons for this rapid transformation of attitude – imagine Gore coming out for gay marriage in 2000 and Dubya merely demurring that although against it himself, he thought it was a matter for individual states to decide – I can’t imagine the tide ever reversing on this issue, despite the Trump Administration’s dreams about turning back the clock. He would be better off, I think, concentrating on keeping his hands to himself.