I think its hard to give an Ultimatum when kids are already involved as they now see the man as the "Father figure". I suggest talking about it,(emphasising the positives. lol) leave the idea in his head and hopefully one day, he might pop the Q.
I think you are right when you say it is a difficult situation when the mother is ready to commit and the father is not. I know that mother truly hopes she can keep her family together. Thanks for the response Lady E.
If your ready to commit, you obviously want something different than what you have. Might be time to move on. If he hasn't popped the question, and doesn't seem like he wants to, then that is probably what he is thinking (I don't want to) Or maybe he is just saving up to buy you a fat rock, and waiting for the perfect moment to ask you. Does he change the conversation when you bring up marriage, or get defensive? i hope thing work out for you. Good Luck.
He never gets defensive but he doesn't go out of his way to change anything either. Being engaged for six years is a long time. I think the word comfortable comes to mind. Maybe it is time to move on...