Do you have a source on a country that legalized gay marriage without social networks? I’m not saying it can’t happen but that would put a dent in my theory. I think Taiwan had it happen before FB but they have what I would consider social networks before we did.
Taiwan was very recent - 2019.
I'll grant that none of these were before the internet but there's nothing uniquely special about gay marriage that required the internet for organising, a huge amount of activism was done in person, just like in various other civil rights movements in assorted countries pre-internet.
So I might still have a point?
Same thing if you think about the #metoo movement, all it took was a tweet for the world to start moving, whereas it took a woman to kill herself in France in 1944 for women to get the right to vote.
[1]: https://en.wikipedia.org/wiki/Same-sex_marriage#Timeline