It's actually losing a lot of ground very quickly. Polls show a pretty huge shifts in young and middle age people towards acceptance of gay marriage. The issue is that those shifts aren't happening uniformly across the USA. The ones that have shifted the least are older evangelical Christians which make up a majority in certain states, mostly Southern. Even then they'll be a minority on this issue within 20 years. If the trend continues I'd put even money on at least half the states passing SSM within 20 years, followed by a Constitutional challenge that will pass the 14th amendment of equal protection. These state amendments are rearguard movements trying to stop a trend.
Long-term trends show a gradual rise in support of gay marriage, whch is the good news.
From
Gallup:
Bad news is a small rise in opposition these last two years. But it remains to be seen if this is a sign of long-term opinion. I think we can be pretty optimistic about it being just a short-term blip. They have happened in the past.
America is divided almost exactly in half and it'll need to be more like two-thirds in favour before real progress is actually felt. So if it continues to be 1-2% a year, that's within 10-20 years.