This bears a lot of similarity to the ‘splitting’ of the Episcopal Church fifteen years ago over the consecration of Gene Robinson. The resulting Anglican Church in North America still exists and is roughly a tenth the size of its parent. The parent Episcopal Church appears to be doing just fine in comparison to other Christian sects, occupying a niche of liberal social values with relatively formal ritual services.
Episcopalian
Episcopal Church( US ) - ~1.8 million members( out of an estimated 3 million American Episcopalians )
Lutheran:
ELCA - ~3.3 million members.
LCMS - ~2 million members.
Presbyterian
PCUSA - ~ 1.4 million members.
PCA - ~ ~400,000 members.
Baptist
American Baptist - ~1.1 members
Southern Baptist - ~14.8 million members.
I won’t entirely disagree with your thesis as it is possible, particularly in the United States, that conservative churches have a better retention rate and liberal church goers slide out of observance more easily. The number of non-religious people( distinct from atheism )has grown enormously over the last 30 years - it is entirely possible, probably likely, that has come more at the expense of mainline denominations.
But I also won’t entirely agree either - as above your estimates of mainline vs. conservative protestant sub-sects in the US are mostly off. The single, but massive exception are the Southern Baptists, a huge denomination that heavily skews those numbers. But If mainline churches do in fact decline faster than conservative ones, it doesn’t seem to correlate with conservative churches winning the ideological fight relative to their own confession. In mainline churches that have split, it looks like the mainline sect is still usually the largest. The Baptists being as above the glaring exception.
When I was young the Methodist Church got into politics, and one thing it decided was that guns were evil, so I split off from it.
But churches having a split is nothing new.
Let’s not use that word "homophobic’ on something which may be simply a matter of doctrine (altho certainly many people who hold that gays shouldnt marry are homophobic, not all are). The Methodist Church used to be one of the “love the sinner but hate the sin” churches, which had no real issue with gays as members.
Altho yes, Paul taught that gay sex was a sin, he also taught adultery, drunkenness, and fornication are just as bad. If you just cherry pick ‘gay sex’ out of that and accept the rest as “OK” then you might be homophobic. If you think* all* are sins- then you might be a blue nose but not necessarily homophobic.
You might also want to consider church attendance instead of membership. My impression is that the conservatives are more likely to attend church weekly while the liberals are more likely to attend church only on special occasions.
It is possible for the doctrine itself to be homophobic.
Yes, but bear in mind that even in just in the US, the broad pattern has been mainline/liberal decline, and evangelical growth or at a minimum stasis. Data below.
I think this is very accurate.
You’re looking at the static membership numbers; I was talking long-term trendlines. The story over the last ~50 years has been
- Mainline denominations that have attempted to modernize, softened moral stances, etc. have been in decline
- Evangelicalism has been rising, though it seems to have plateaued in the last ~10 years (with Mormons showing the same pattern)
- Charismatic/Pentecostal churches growing even more than evangelicalism; they are something of a mixed bag, but on the whole the lean conservative on moral issues.
http://www.bradbridges.net/2016/01/14/the-shifting-denominational-trends-in-the-american-church/
Search all over, you’ll see the same basic pattern.
Episcopal church membership in 1966 was 3.6 million. They’ve lost half their members in 50 years, with no end in sight. Episcopal Church Continues Downward Trend According to Report | Church & Ministries
The mainline PCUSA has been dropping for decades Presbyterian Church (USA) - Wikipedia, but the evangelical PCA, which split off in 1973 about ordaining women, has grown 10x since Presbyterian Church in America - Wikipedia
The ELCA had 5 million members in 1987; they’ve lost over 20% since. ELCA Has Biggest Split in American Church History | VirtueOnline – The Voice for Global Orthodox Anglicanism AFAICT the other Lutheran churches are declining, too, but not as rapidly.
I can’t think of a single liberal/mainline denomination that hasn’t been in long-term decline. Evangelicalism has been much stronger (though the more recent trend has been toward non-denominational evangelical churches) Thus, the most probable forecast is that the liberal UMC branch will continue to decline. The evangelical wing’s fate harder to predict.
My parents are practicing Methodists. The individual churches voted and those results were tabulated up to reach this decision. I know that people hoped that the church would not split, but that they also voted their conscience. My parents church elected to support LBGTQ members.
On the personal side, my parents have elected to leave churches that did not allow LBGTQ in the past. I have to admit that I am proud of them from time to time. 
I think it was one of Robert Heinlein’s characters that said “Religions have schisms as naturally as a cat has kittens”
The denomination I grew up in, the Lutheran Church-Missouri Synod had a major schism in the 1970’s. The congregations that left at first had their own smaller synod, then merged with two others to form a new group, the Evangelical Lutheran Church in America. This latter group is one I ended up belonging to, then switched to the Episcopal Church. The ELCA and the Episcopla church are in what is called communion with each other. They maintain seperate group identity and history, but clergy can serve in either church. In fact, the cathedral church I belong to has had an ELCA pastor serving it for some years, along with other Episcopal clergy. Rev Goerge was a military chaplain for a long time, now, semi-retired, he does some hospital work. He’s really cool, a West Point graduate of all things.
I grew up in the Methodist church in the deep south. I never paid much attention to church doctrine, our church was pretty apolitical (in hindsight, a consequence of everyone assuming everyone was on the same page). So the first time I saw some of the church’s updated policy positions in the late 80’s, I was shocked how liberal it was (relative to my understanding of the majority of the community’s attitudes).
So, color me unsurprised to see the Methodist church split between cultural hardliners/softliners, and unsurprising to see the Southern branch go the way of third world superstitions about sexuality and gender.
To be clear, I left religion behind a long time ago and I don’t really care what churches do. It’s just an interesting reflection of what’s happening in larger society.
You seem to be imagining a Venn diagram where there’s no overlap between liberal & Christian. There is a great number of people who live in that overlap. The purpose and need of the church for them is to hear an explicitly Christian message and spend time with explicitly Christian people.
It’s the exact same that is true for conservatives who are also Christian -there are plenty of places in and around that aren’t the church that are pro Conservative and anti-LGBT+. If being anti-gay is their only reason for going to church - they might be churching wrong.
Southern Baptists aren’t usually considered mainline churches. They’re Southern Baptist.
It seems to be a step in a direction (which is a good thing). Trying to straddle the line was not working for anyone. There had been a lot of friction for a long time with a lot of people caught in the crosshairs (e.g., if you were a congregant and knew that your pastor could lose their job and your congregation could lose its pastor if they officiated your wedding, you might not ask just to keep the peace. That’s a sucky place to be and a lot of consequences no matter what your decision when all you want to do is get married. Now, that’s gone.)
Many of my colleagues are some of the most liberal people I know and are extremely (Christian) religious. There is tremendous overlap between liberal and religious, when it’s supported and allowed to thrive
Do you now belong to a more or less conservative church?
So, pretty much everyone prior to say, the late 20th century, was “homophobic”? Mind you there were some gay people that didnt like the idea of gay marriage.
I dont belong to any.
In many ways the new Pope is pretty liberal, as Popes go.
Right, some of the comments seem to assume the opposite but the idea is to *spin off *the “traditionalist” congregations.
Yup, that’s one bit that is often kind of glossed over. Part of the proposed separation settlement seems to be recognizing that situation: https://www.cnn.com/2020/01/03/us/united-methodist-church-split-proposal/index.html
Ineed, never mind Methodist, that’s a characteristic of American Protestants in general.
“as Popes go.” 
Important caveat (doctrinally Francis is barely distinguishable from Benedict or John Paul)
Yet non-denominational Christianity tends to be some variant off of Southern Baptist. Around here in the Bible Belt, I do tend to think of Southern Baptist when I think of generic Christianity. When I would go to other denominations, they seemed like variations on Southern Baptist, just with something extra. (The sole exception was the Catholic college group I went to.)
This board is actually the first time I learned that this was not what people meant by “mainline Christianity.”
Most non-gay people, yes. Just like through most of American history, most white Americans were racist.
True, though the African and Asian churches were strongly supported on that vote by conservative American UMC members and churches, such as the Wesleyan Covenant Association.
It’s probably safe to say that most UMC churches and members in the US are on the liberal side, but definitely not all. The conservative UMC churches in the US skew towards the South, but I’m sure that there are some conservative ones in the North, too.
Last year, after the Special Conference vote where the conservatives managed to get the “Traditional Plan” approved, it looked that way to me, as well; it appeared that liberal UMC churches, which wanted to be welcoming to LGBT people, were going to be the once forced to leave the denomination. (Just to note – at that time, there was no “splitting” yet happening, or even announced; it was all still in the hypothetical.)
But, yes, this week’s agreement suggests that it’ll be the conservatives who are leaving, so that surprised me, too.
Also – there is an agreement, but it has not yet been voted on by the full church (the news reports don’t always make this clear). That full vote won’t happen until the upcoming General Conference, in Minneapolis in May. The news stories make it sound like the agreement will likely be approved, but it’s not a done deal yet.