I guess the Prebyterians have joined the ELCA and are ordaining homosexuals:
From a purely pragmatic perspective (of course, there's the whole thing about this violating the immutable will of God, but I think we all agree about that part!), I sort of don't get why they did it.
I mean, most mainline Protestants don't functionally believe in God. If they did, they wouldn't believe that the truth was a wax nose that they could mold as you like (See: Hanson, Bishop Mark). They would be reverent about how they handle the Bible. But they're not. So the only thing I can conclude is that they functionally don't believe in God as an ontological reality.
So, if you functionally don't believe and your goal is to simply perpetuate the institution (for some reason- I have some ideas), then why do something to make the institution unpopular among people who are really comitted to it? I mean, do you think that the marginal, already secularized people in the PCUSA are really ones paying the bills? No, of course not, it's the firebrand, Evangelical types (of which there are many!). So, why alienate them for the sake of 4% of your members? It makes no sense.