Believing that people should keep their vows, oaths or promises is being delusional? Assuming your statistic is accurate, that means 50% percent of couples do not know how to pick a marriage partner. If you expect your future spouse to "fall from grace" then by all means get a prenup.
Even in some of the best case scenarios Clinton happens. One partner cheats or one parter develops a gambling habit or something like that. Why should the innocent party suffer?