Question

A person who doesn't believe in the institution of marriage one word substitute

Answer 1

A person who doesn't believe in the institution of marriage is called a 'misogamist'.

Answer 2

To explain the person "who does not believe in the institution of marriage"  or the one word for the person who doesn’t believe in marriage is called Misogamist.

EXPLANATION:

Misogamist is happy not being married because of a rough childhood or would have undergone a worst experience about marriage life. Their mindset is not to commit themselves with opposite gender in the name of marriage.

A Misogamist would want to convince the other person against marriage because of his strong beliefs against it.