OP, the reason why he doesn't want to marry I suspect is because he knows he has a lot to lose if your marriage went downhill.
Right now if he was to break up with you or you with him all he had to financially lose is what, part of a house? His pension, savings etc I expect are safe and if there was a will he could easily change it. If you were married and either of you wanted to divorce then things would be more messy and he would have more to lose.
It's easier for him to put your name down in his will, insurance, whatever because if you seperated he could sort everything out without losing anything.
I suggest forget marriage and start creating savings of your own in the event of separation. You might never want to seperate from him but if one day he does then you will have no say in it and you will be financially screwed.
There have been threads here in the past where posters said their husbands/partners were the kindest, loveliest, most respectful men on earth, they would never cheat, trusted them 100%, etc etc...Guess what? They had affairs.
Things change all the time, right now you might have a good relationship (apart from the proposal issue), tomorrow you might not.
You need to have a plan b. Even if you were married you need to have a plan b. Every woman should. My mum learned that the hard way when her marriage fell apart.
Good luck.