I think a lot of people who were on the fence about same sex marriage, or who were worried about the effects of legalized SSM on the greater society, have been convinced by the evidence accumulated thus far that there's no real reason to oppose it. The places that have legalized SSM in the U.S. seem to be doing OK and the net effect on heterosexual marriage has been approximately zero. This counters all the propaganda from the anti-SSM camp about all the awful things that would happen in our society if SSM were legalized.

Approval of SSM also inversely correlates fairly strongly with age. Old people tend to die.