Unfortunately many religions do not allow for cohabitation before marriage. It is seen as a sin, so many if not the out right majority don't do it. Not that i dont agree with you, but your advice will not apply to the majority of people.
Secondly i am in just such a situation that you think is unlikely.. Dated my wife for 4 years before we got married. Everything before the marriage was great, but the moment i put a ring on it and said vows, literally everything changed. That. Very. Week. I didn't see it coming. Been trying to fix us for years. Not going well, so i am close to divorcing her now.. Edit to add. Unless you are talking about living with each other, as we didn't do that. It seems Cohabitation is when the real mask comes off, not the couple who dates but lives separately. I honestly wish i had lived with her first as i would not have married her knowing what i know now..
This is why i come off like a rabid redpill. I see marriage as a trap, a nightmare that can be avoided and will do my utmost to prevent young, virile men from making the most expensive mistake of their lives. There is no way to tell with someone who is an expert at faking their emotions and behaviors. It has nothing to do with the male being stupid or not, some people are just expert liars.