Look at the following graph of divorce over time. [...] Again, note the collapsing marriage rate from the early 1970s. Once people realised that marriage wasn't enforceable, the marriage rate collapsed.

I'm not sure what point are you trying to make with these graphs.
If people were allowed to make binding pre-nup agreements that penalized adultery would there be more marriages? Less divorces? More happiness?
None of these things seem obvious to me.