You enter into an exclusive, romantic, sexual relationship in good faith. You have the intention of “forever,” in spite of knowing the statistics on marriage casualty. I certainly did, and I completely believed my partner did too… each time. (In July we celebrated the 31st anniversary of our third marriage.)
In the 17th, 18th and 19th centuries, marriages lasted between eight and nine years. That was how long it took on average for one of the partners to die. In the 20th century as people started living longer, divorce gradually replaced death as a way of ending a marriage. Up to 1970 infidelity was the only grounds for divorce in Canada. As that became unworkable, divorce laws were liberalized. At the same time increasing numbers of committed couples simply did not legalize their marriages.