Ideally "reality" hits a couple long before they say; "I do" if you referring to them knowing each other's "authentic self".
Hopefully they've seen each other during stressful and unhappy times as well as witness the ranges of their temperament.
It's also important to make sure one has selected a mate who shares their same values, wants the same things for the marriage, naturally agree on how to obtain those things, and last but not least have a mutual depth of love and desire for one another.
Anyone who expects their marriage to resemble a "fairytale" is likely to be very disappointed after they return home from their honeymoon.
The two leading causes for divorce is a) Selecting the "wrong mate" for oneself. b) Getting married for the "wrong reasons".
The first happens when one has not figured out (who they are), what they want and need in a mate for life BEFORE they pursue relationships with others. They're likely to let impulsive connections and happenstance dictate their relationship choices. After saying "I do" they attempt to change their mate into the person they (really) want.
There are only two ways to experience joy and peace of mind in relationships: We either get what we want or we learn to be happy with what we have.
The second happens when people allow (circumstances) to influence their decision to marry. Examples include having reached an "age goal", being given an ultimatum, there was an unplanned pregnancy, all of their friends had gotten married, or someone joined the military and was about to be shipped out and they wanted to know there would be someone waiting...etc
A marriage based upon circumstances rather than love is likely to fail.
A third cause is some people get married before they have their ducks in a row. They have not completed their education, do not have an established career path, they're drowning in debt and still elect to have to have children with the belief "somehow" things will work itself out.
He who fails to plan is planning to fail. The wedding is just one day.