Here’s my random, unsolicited opinion:
I doubt porn is the cause. Porn might be a comorbidity (what I’d put my money on), or maybe a symptom (I doubt that, too), but the idea that it’s the cause is pretty odd to me.
I think marriage is fundamentally an economic institution. People got married because it made economic sense. You needed a partner to rear children with, assuming you wanted children at all. There’s all the stuff about love and companionship and all that, but there’s no reason why we need to fixate on one particular arrangement. It was about partnerships of families.
I think marriage made a lot of sense in small towns. It makes sense for large, political dynasties, for the old wealthy, etc. It doesn’t make so much sense for the young middle class today.
This is my rough, unvarnished opinion. Would love to hear criticisms, refinements, etc.