I agree with the top post more than the second, since I tend to take a dimmer view of how Christianity affects society and social well-being, although that is possibly colored by my rather disproportionate exposure to (1) people who had a traumatic experience with Christianity and/or (2) academics in academia subculture where any kind of traditional religiosity or even God-belief is fairly unusual and everyone seems to get by just fine with viewing the world within an ethical framework and relating to each other. (And I count myself among those atheists in academic subculture who fit in just fine in that regard. Although arguably one downside is that people in academia and other secular progressive subcultures tend to fill a religion-shaped hole with certain religion-like ideologies, which is a trend I don't fit in with very well.)
I basically want to second all the rest and mention that I appreciate seeing someone else besides me say it. (Perhaps I just don't have as good a way of saying these things, but when I try to express some of them on Tumblr, in return I get a good bit of flak.) Yeah, the gay marriage debate is basically yesteryear's major culture war battle and represented how that old culture war really revolved around Christianity vs. secularism (with each waging war in a different way against Islam). Yeah, it mainly got supplanted by trans issues -- that's what both social conservatives and social liberals decided around the same time to focus on, once gay rights by and large became a done deal on the legal front and was established in the hearts and minds of a solid enough majority of Americans. I'm not sure I'd call gay marriage "vanishingly rare" from my experience (one of my current colleagues is in a same-sex marriage, and I know other academics in such as well, for instance), but it doesn't seem to be something the whole gay community has jumped to either. Yes, trans issues are different from gay rights issues in a number of key ways (this is the part I get in trouble for, and I don't want to suggest it's exactly what you're saying because it isn't), including requiring the appearance of greater social shifts (I would go further and claim, actually requiring greater social shifts).
The trans issues culture war battleground is not a proxy for Christianity vs. secularism, even though some parts of the anti-trans side comes in a conservative Christian flavor and is held by conservative Christians, and even though the liberal side seems determined to brand the debate over trans rights as the Gay Rights Debate 2.0 (and perhaps the conservative side does this as well, although I doubt it since it would be disadvantageous branding given that they lost the gay rights debate?).
Yeah, I've always said that abortion is always going to be a contentious issue and I don't see how that will ever change, because it involves a legitimately deep philosophical/ethical conundrum that can't be boiled down to getting rid of "hostility to, discomfort with, and resistance towards accommodating people who are different along some axis" -flavor conservative ideas that inevitably will eventually decline, as applies to gay rights stuff and many aspects of trans rights stuff.
Probably if (some version of) christianity were to again become dominant in the US then different kinds of Christians would go back to being at each other's throats, as opposed to being more or less friendly in the face of broader secularism
I don't know. Christianity was still fairly dominant when I was growing up, in the days before New Atheism had begun to really gather strength. It was certainly quite dominant in the '80's as well. It's true that if you go back far enough -- to a century ago, say -- there was a lot more friction in the US between Catholics and Protestants than we see now. But throughout most of living memory, I'm not sure such conflicts have been very prominent.