I think inkydinky talks a lot of sense. You may not need his money, but if the children move against you when he dies, say in 20 years' time, the emotional pain and trauma will still be huge - even if you do have a house to go to. If you love the house, you would be most upset to be kicked out of it, however much money you had.
Marriage is about security as well as love. It is odd that it is often seen only as security for the woman. Why is it not an issue for him also? What does he risk by not marrying you? These 'worst case scenarios' are significant for him too.
He could totally make his will water-tight, whilst making provision for you to live in your shared home for the rest of your life. I agree there is anxiety about what happens if he died before you and you were married. Maybe he thinks that his children could be done out of an inheritance after he goes. This happened to my cousins and their mother's widower. My aunt and he got 'mirror' wills drawn up together, so that the right proportions of their joint assets were shared between their respective children. When she died, he inherited her assets; he made a unilateral decision what cash to give her two children as an inheritance, changed his will and has just bought a big property with his daughter which she will no doubt wholly inherit when he dies. My cousins have no claim, despite their mother's wishes for them to have a fair share of her assets.
Marriage obviously does mean an awful lot to him. Otherwise he would not be so strongly against it. He must think it is extremely significant. It sounds as if he is being disingenuous, at the very least, saying that it ' doesn't mean anything'.
I can understand why this niggles you, because his reasoning does not add up.