Here's a Google Ngram graph of usage in books of the terms "gay marriage" in red and "homosexual marriage" in blue from 1800 to 2008. The terms were essentially nonexistent until the early 1970s, after which there were a tiny, relatively stable number of references to "homosexual marriage" for two decades. Then there was an inflection point around 1994 and another one around 2003. (Methodology notes: The graph above reflects Ngram's default three-year moving average smoothing. If you turn off smoothing, the inflection points appear a little later than when smoothing is on. Of course, books perhaps lag behind other media because of their longer production cycles.)
I'm fascinated by the mechanics of media muscle reflected in the two inflection points. Here's a topic that had interested almost nobody, straight or gay, for, roughly, ever, yet then in two stages becomes a cultural obsession.
My vague recollection from following the news at the time is that the 1990s one was largely due to a concentrated push by the New York Times in the early to mid 1990s. The Times' front page was obsessed with gay genes and gay this and gay that. Perhaps the second inflection point was largely created by the Times as well. In 2000, Richard Berke, the NYT's gay National Political Correspondent boasted to the National Lesbian and Gay Journalists Association:
Now it's like, there are times when you look at the front-page meeting and ... literally three-quarters of the people deciding what's on the front page are not-so-closeted homosexuals. ...Now, a theory I've long entertained is that the gay marriage brouhaha reflects a fundamentally healthy movement among gays to push more restrained lifestyles on themselves after their catastrophic debauchery in the 1970s following Gay Liberation caused the AIDS epidemic.
But, it's just expressed triple bankshot-style through today's Who? Whom? conceptual vocabulary.
These days, you see, it doesn't pay to upbraid your own kind to behave in a more traditionally moral manner, and perhaps apologize to society in general for your misdeeds and promise to act better in the future so as not to cause another horrific venereal disease epidemic. That's so ... Victorian! What did the Victorians ever accomplish? (I mean, besides building all those to-die for Victorian houses in the Castro district?)
We don't live in the Victorian Age, we live in the Victim Age. So, gays admitting that AIDS was their own fault was never on the table. So, AIDS had to be ... uh ... Ronald Reagan's fault!
Similarly, gays can't admit that they need moral reform even when they realize it themselves. Instead, they have to be victims of oppression denying them the right to something they had never noticed they lacked in the past.