DP and I have discussed marriage several times and still end up at the "I dunno" point.
The thing is, marriage at our age (and with 1 marriage under his belt and 2!! under mine) just isn't as important as it used to be when we were younger, in terms of security, children etc. But it would make some things more complicated now- we both have children, we both have assets we want to leave to our children- so we'd have to draw up something to ensure that that happens.
On balance, it's me that doesn't want to get married, for the sole, and shallow, reason that I feel it's a bit embarrassing, to get married for a third time. (for me- I have no judgement on anyone else!)
But of this I am absolutely certain- I love this man more deeply and passionately then I did the previous two. I want to spend the rest of my life with him and the thought of ever being without him horrifies me.
So me not marrying him is, to me, irrelevant. It's the relationship that counts.
However- if marriage were important to him, for whatever reason, I would marry him, if it made him happy. And I think that's the crux of the matter- would your DP marry you, to make you happy? Is the marriage ceremony unimportant enough to him, and your relationship important enough, to do that?