The United Methodist Church has for years gone the way of the mainline Protestant denominations generally in accepting the conclusions of critical biblical scholarship (rightly so, I would say), and also going easy on the historic marks of orthodox faith. That is to say that, beyond questions of biblical authorship and so forth, mainline Protestantism has been very open to reinterpreting Christian language in ways that sounds orthodox but really isn't. For example, one never knows if a mainline Protestant preacher means the same thing as the people in the pews when they say 'God.' While the parishioners may think of God as three persons in one being, or simply a stern but kindly old man in the sky, the pastor is crossing his or her fingers as they say 'God,' meaning instead something along the lines of pantheism, panentheism, a force, or a symbol of the highest good. Or, who knows, maybe they actually mean the trinitarian God of the ancient ecumenical confessions. The same goes for terms like 'incarnation' or 'resurrection.' What a mainline minister means by those words is often very different from what a conservative evangelical preacher would mean, and the former will not go out of their way to make sure you understand the difference.
The most notable instance came in 1844 when Northern and Southern Methodists divided into different denominations over the question of whether a bishop could own slaves. Many historians cite it and other big denominational divides around the same time as signs that the U.S. was heading to civil war. (Taking stock after a season of disaffiliations, UMNews)
I do not think it is correct to compare the current split with that leading up to the US Civil War in the 1800s. Perhaps chief among the many reasons for that is the fact that something very fundamental is involved this time around. In the 1800s it was a very serious matter indeed, that of whether it was possible for someone to be a good Christian and enslave other human beings. Today the matter most loudly under discussion is whether to extend to lgbtq+ folks all the rights and privileges of membership, including marriage and ordination. Certainly that is also a human rights issue. A key difference, I believe, from the 1800s split and this one, is that in that of the 19th century everyone still believed fundamentally the same things about God, the Bible, and the Christian faith. Critical biblical scholarship was certainly on its way, but it was not a significant issue at the time. Even when the divided churches then reunited in the 20th century, when such scholarship was in full bloom, the two denominations were not that far apart doctrinally. This time it is different.
The human mind is flexible in matters of religion. People are capable of believing in historic trinitarian Christianity and also understand that the Bible is not a word-for-word inspired text. Furthermore, people are also able to come to a place where they see committed same-sex relationships as holy within the framework of that essentially orthodox Christian faith. So, while I think it is possible that in a couple of generations, or even less, most evangelicals will say that of course same sex marriages are valid and good, that does not mean that they will let go of orthodox claims about God, Jesus, and the Christian faith. Thus, even if the newly formed Global Methodist Church does someday take the stand I've described, they will still be far apart from the United Methodist Church, assuming the latter continues on the course of welcoming alternative interpretations of fundamental Christian doctrines.
Then again, I don't know the future. It would be very nice if all this energy wasted on debating the particulars of religion, or arguing against the rights of human beings to live their own lives peacefully in the way that they wish, could be spent on solving the real problems facing our species. Climate change comes to mind, as does racism, militarism, and so much more.
