If he is already getting everything on a plate, a committed partner, a sexual relationship, children, someone to split the bills with, emotional support and a family life, all without making any legal or lifelong commitment, why would he change anything?
I know this does NOT apply to every couple or to ALL men. Some people genuinely do not want to marry and both are happy with that. But if one person wants marriage and the other keeps putting it off year after year, are they really going to change when they are already getting everything they want?
Why would someone buy the ticket when they are already getting the whole show for free?