I have no idea where you get that acceptance of same sex marriage arises from Christian ideas. Because in my personal experience it really doesn't. I live in Italy and a lot of discrimination against gay people (me included), and a big part of the reason why we don't get actual marriage, is catholic rhetoric. Most of the homophobia I've experienced in my life, anecdotally, has been from devout christians: the more devout, the worse the discriminantion. So although I have no data on hand, I have a very strong suspicion that Christianity has nothing to do with it. If anything, it seems like at least in Europe, the more secular countries tend to get better rights for queer people and they get them faster (I think the first European country to get gay marriage was the Netherlands, back in 2001. And they're one of the least religious countries in Europe)
It’s hard to see if you limit your view to Europe, because Christian morality is just “in the water” there. I’m from Asia, where same-sex marriage is not accepted anywhere except Taiwan. It’s not accepted in Buddhist Thailand, Hindu India, Muslim Pakistan, atheist China, or Shinto/Buddhist Japan. It’s hard not to notice that the places where same-sex marriage is accepted have something in common: they were Christian for more than a thousand years.
It’s not just “secularism”—the absence of belief in God or absence of organized religion. Japan and China are extremely secular countries, more so than anywhere in Europe. The notable thing about the Netherlands seems to be not that they’re secular, but that they also were a hotbed of the Protestant reformation. Until the 20th century, the majority of the Dutch population was Calvinist. A few years after the Netherlands legalized same sex marriage—and long before most of the rest of Europe did so—Massachusetts became the first state to legalize same sex marriage. Massachusetts was founded by fundamentalist Calvinists (the Puritans). Even though American Calvinists and European Calvinists split (geographically) 300-400 years ago, their version of “secularism” evolved in a very similar direction, and quite differently than the “secularism” of Asian countries. It’s really hard to say that’s just a coincidence.
