I think it was because homosexuals decided to risk being out. I’m not saying they didn’t get harshly treated when they dared it, plenty did, and it still happens of course. But once people had a chance to see the reality - that they are just regular people with a different sexual orientation I think things began to change.
Ultimately people’s opinion changed because they knew someone who was gay, who was a good person and deserved a life free of persecution and bigotry.
Regardless of religious influence, (whether waxing or waning, whatever flavour), most people’s deepest beliefs seem to involve some measure of ‘live and let live’ along with a healthy dose of ‘judge not lest you be judged’, thrown in.
In times of less openness people would discover, way late, that their ‘friend’ was ‘that way’. How would it make you feel about all the homophobic things/jokes you’d made? How would you feel about yourself if someone close to you felt you couldn’t be trusted to know this about them?
Things changed for the same reason interracial marriages stopped being illegal, segregation was ended, and beating your wife/children became a crime - because we evolved, our consciousness was raised as to the unfairness and cruelty of such things.
Just because something traditionally ‘always was’ is such a silly argument, for this very reason.
If a vast majority of people feel it’s time to shift this, then it’s time, no matter how much it sucks to be in the minority, holding fast to the more traditional view. When women sought voting rights, black Americans sought equal rights, etc, the traditionalists with the more conservative view always swore that these things would surely produce the downfall of society, as we know it. Fortunately these lame arguments are recognized as such today because, of course, they were wrong then as well.
Gay marriage only affects you if you are gay, all attempts to assert otherwise are nonsense. If Kim Kardashian’s fake profit seeking marriage for TV ratings and fame doesn’t diminish traditional marriage, I’m not seeing the threat from Adam and Steve getting married.
I do have empathy for people who were once but a tree in a thick heavy forest of others, on this issue, and now suddenly find themselves a single tree on a vast open plain. That’s gotta suck, to be sure.