Presbyterians vote to endorse SSM

[DC Al Fine]

Also, this change requires approval from a majority of the 172 regional presbyteries next year, so it's not a done deal quite yet.

Are the southern PCUSA branches more conservative than the northern ones? 

I wouldn't know personally, but I'd assume that to be true.  The PC(USA) already lost a number of churches after they removed barriers to ordination for people with same-sex partners.  At this point, any church that wants to leave the church for it not being conservative enough has probably already done so

You'd think that, but it's surprising how strong inertia can be. My local United Church (to the left of American Episcopalians) is having a schism vote some 15+ years after they started blessing same-sex unions

[DC Al Fine]

To those in the know: How much of the "mainline Protestant decline/conservative and/or evangelical Protestant increase" can be attributed to differences in birth rates between the two categories of Protestants?

It can't be much. A difference in birth rates could explain a slow drift in membership statistics across many generations, but we're not talking about a subtle change here; we're talking about a nearly complete obliteration of many of the mainline denominations, in the case of the PCUSA losing close to half its members over 30 years.

You have to get extremely conservative for birth rates to make any difference between denominations. Sects that eschew birth control have seen rapid growth. Ex: There's a Psalms-only Presbyterian church in my hometown that has surpassed my own church almost entirely off the women have 8 kids each.

[DC Al Fine]

I had a PCUSA minister give the invocation at my college graduation ceremony. It was... strange. She started off with the phrase "O great spirit..." at the beginning of the prayer and her pronoun usage switched somewhere in the middle from being directed at a single entity to being directed at the people in attendance. It was really bizarre and I wasn't sure where she was talking to God or us. I'd also been told by people that the PCUSA church on campus was more or less post-Christian when it came to their concept of the nature of God. Their idea of God was only loosely tethered to an idea of a personhood at all, let alone Trinitarian.

There's a pretty huge variation within mainline Presbyterian churches. You can still get a conservative Reformed message in one, while the one down the road preaches the post-Christian stuff you're describing.

[DC Al Fine]

Good news.  It will be interesting to see if the PCUSA's decision intensifies the pressure that the United Methodist Church faces from within to do the same.

Perhaps, but they won't succeed.   Although it's probable that opinion in the UMC continues to march to the left within the US, most of the growth of the denomination is outside the country, in more theologically conservative regions, which are therefore getting a larger and larger share of UMC General Convention delegates to vote against any liberalization of UMC policies on LGBT issues.  If anything, this sort of decision by yet another mainline Protestant denomination may make UMC members even more agitated and prone to a schism, although I don't think the denomination is there yet.  (It would be very interesting to see a "schism from the left".)

UMC seems destined for schism of one sort or another. The progressive and conservative wings are just too evenly matched.

As for the PCUSA, I was talking to my PCA pastor today. He said that a few dozen congregations saw the writing on the wall and jumped to the PCA a few months before the vote, but there's no word of quitters now.