Would any “Eastern European” (formerly Communist) country vote in favor? Slovenia, Estonia, and the Czech Republic would seem the most likely, but Slovenia turned it down in a referendum a couple years ago.
South Africa is a country with gay marriage (thanks to the court) where I’m guessing a solid majority of the people would vote against it.
Asia is a real mystery. No country has legalized it yet. Polls have generally shown majority support in Taiwan (which—along with Nepal—was supposed to legalize it by now—what’s the holdup?). Thailand would be another good bet for majority support.
Oh yeah, what about Northern Ireland? Non-Protestants would likely vote in favor, not sure if it would be enough though.
Czech and Estonia are the two least religious countries in Eastern Europe, by far. It would pass with 50–55%.