Why did the overton window on same-sex marriage shift so quickly during Obama’s first term?

[RINO Tom]

As someone who lived through that era at an old enough age but wasn't quite as into politics ... with hindsight, I think a way larger percentage of people didn't actually care.  There were SO many Democrats who "supported full-benefits civil unions," because that's what their standard bearers supported, so that's what ~Democrats supported~.  Obviously, they never really had an objection to same-sex marriage, at least not a very strong one.  Additionally, I think it was actually a minority of Republicans, too, who really cared that much.  You had your die-hards, sure (probably 30-40% of the party at that time), but there was a big chunk that just repeated the party line of opposing it and would have DEFINITELY compromised on that issue if it meant advancing other conservative causes.

Once you had a sitting Democratic President openly support it, it was a free pass for nearly all Democrats to support it, most independents to support it and then those Republicans who didn't really care to drop it, if not support it openly.  That's a recipe for a very quick change.