I think it was a case of middle-aged Stan thinking the Sexual Revolution was just a passing fad (the 20's and 30's were pretty wild too, before the Great Depression hit) and that MJ was just a curiosity who would quickly become obsolete and Peter would go back to the traditionally acceptable Gwen Stacy. For all his claims in later years, he didn't seriously consider MJ to be a long time love interest for Peter in the 60's. Her immense popularity was the only reason he kept her around, by making her date Harry, and even then he started using her sporadically. It was when an actual teen Gerry Conway took writing duties and had no idea why Stan insisted on boring old Gwen being the female lead when MJ was right there that things started looking up for Mary Jane.
The Riverdale show did JUST THAT just a couple of years ago with cementing Veronica as endgame for Archie (that's an actual line from the show) and they even recreated the ASM #122 epilogue with Archie and Veronica (not surprising since the head writer of the Archie-verse shows is Roberto Aguirre-Sacasa who is a big MJ fan and wrote a pretty good MJ story during the Back in Black era). Betty too got a "Dark Betty" persona revealing there wasn't all that met the eye when it came to the girl next door.but I honestly don't think that our society has evolved all that muchthat you couldn't subvert the old Betty and Veronica stereotype the same way that Conway did in the 70s when he showed that there was more to MJ than meets the eye.
TBF that's only because double the number of people watch the Spider-Man films as those who watch Ant-Man films. MCU Peter/MJ are also incredibly popular among the Twitter crowd so it goes both ways.