I think we're up to 59% pro as of now, with broad demographic trends pointing toward further shrinkage of the anti-group above and beyond the natural attrition that comes from it no longer being comfortable to express bigoted views.

http://en.wikipedia.org/wiki/Public_opinion_of_same-sex_marr...

This is an issue where public opinion is changing quickly, as it often does when oppression becomes recognized as oppression. I'd recommend taking the limit of the function as yr->2020.