Again, note the collapsing marriage rate from the early 1970s. Once people realised that marriage wasn't enforceable, the marriage rate collapsed.

I don't think that's the whole story. Marriage rates were declining in the 1970s and 1980s even in countries where divorce wasn't introduced until later, such as Ireland or Malta.

(And intuitively, I'd be less reluctant to do something if it was easier to undo it, though YMMV.)