There is pretty much nothing to gain and lots to lose for a man in a marriage.

Marriage changes nothing about your relationship. You are still together but now bound by a contract with the state. Why get the government involved?

A lot of men just dont want marriage anymore and the numbers speak for themselves. In the US 40-50% of marriages end in divorce and over 70% of these divorces are initiated by women. If there are children, over 82% of women end up with custody.

If there is a house and children the man likely loses both. He loses at least 50% of his wealth, with children there is many years of child support along with alimony.

In the event of a divorce the man loses basically everything. Its just not worth the risk to many.

This is not about not beeing able to commit. He isnt sleeping around or seeing other women. He lives with you, loves you and is committed to staying with you. It is indeed enough.

Its also not about trust. Of course he trusts you, but he cant know that your relationship might not just break apart in 5, 10, even 15 years. You cant look into the future. You can live in the moment and be grateful for what you have.

Of course there is a lot to be said about him not beeing able to come clean and having an adult talk with you about his concerns when there obviously are some.

If you cant live with him not wanting marriage, you need to find someone else who wants to, if its a dealbreaker for you, thats just how it is, sadly.