I think you are right about the potential for division of the country by the gay rights issue. The other issue that could do it is abortion. Any issue for which advocates find justification both in their personal moral codes AND in its effect on the lives of individuals has that explosive potential.
I could see these and other issues causing enough division to bring us to a civil war, not of exactly the same nature as the one over slavery but enough of a conflict to be labeled a real war nonetheless, with violence involved. In my last-days novel that I'm working on, temples in California are being bombed and Mormons are emigrating to Utah in droves, driven out much like in Missouri or Nauvoo.
From my perspective, it's all really clear what lies in the future, for those watching for signs and comparing them to history. Even though the battle isn't technically at the national level yet (like it soon will be), I keep reading stuff like what gay man Andrew Sullivan wrote today on his Atlantic Monthly blog in a post titled "The Mormon War on Gay People": "What we have just witnessed is a trial run for much larger ambitions. If we don't resist this now, we will not be able to resist it later."
Clearly Book of Mormon–level social cataclysm is coming our way. It's taken a good 40 years for the devil to advance his campaign of sexual revolution this far since he started it in earnest in the 1960s, and hopefully it will take another 40 years before the bottom really drops out, but I suspect the next twenty years aren't going to be as fun for us Mormons as the last twenty were. Somewhere during the Romney campaign, the tables turned on us, and then the Texas polygamist thing didn't help. Now with the gay agenda galvanized against us, we're headed down the other side of the mountain into shadow again, as far as our public image and place in society.
Another interesting piece of the puzzle has been to watch how worldly Salt Lake City proper has become, where the church is now actually shrinking and selling off meetinghouses. Former SLC mayor Rocky Anderson's "worldly morality"—he's definitely got his passionate moral causes, but they're far from what God's prophets are saying—was both a symptom and a cause of this trend, which for me was recently illuminated by an interesting blog post written by a virulently anti-Mormon, pro-gay returned missionary about feeling unexpectedly at home after having just moved to Salt Lake:
Here's my thought about Salt Lake City captured in a passage from my novel Kindred Spirits:
While Eliza disliked Utah County for its excessive Mormonism, Salt Lake City gave her a creepy vibe for the opposite reason. The summer after she'd graduated from high school, she'd briefly fallen in with a group that liked to go to clubs and parties in the big, bad city. Eliza had glimpsed some wild partying, which seemed all the wickeder because the people were all either non-Mormons rebelling against the dominant faith or Mormons rebelling against their own religion.
If she remembered right, one of the prophets had said that Salt Lake would eventually become the world's wickedest city, and it wasn't hard to imagine. As the Book of Mormon showed, the evilest people were those who knew the truth but then turned against it. She preferred Boston, where people were worldly and sensual because they generally didn't know much better.