Ju222 I am so sorry that your partner has done this to you. He knows what you want and has strung you along and now revealed that he doesn't seem to want what you want.
Do you want a big day with family and friends all celebrating your marriage or would you be happy with eloping or something very simple.
I think you need to work out why you are putting up with this, have put up with this for 17 years. Why have you allowed this man to put his needs ahead of yours for so long?
I think do what you want. If you want to get pregnant, tell him that this is your plan, he can leave or he can get you pregnant. Whether he marries you or not, well, if you cannot have a child with him without marriage then you do need to know when this marriage will happen. If it will not happen (which is what he is telling you now?), then you know that staying with him means no children and no marriage. The opposite of what you want.
I know what you mean about ultimatums. I had a similar situation but after 3 years not 17. I said to my then boyfriend that we were breaking up because we wanted different things, I wanted marriage and a family and he did not appear to want that.
After we broke up dh arranged for us to see a counsellor together, his choice, and we decided to get married.
This was the second time that me and dh had broken up because he was not ready to commit. After the first time I took him back expecting him to know what I was hoping for! But after a year or so there was no development and so I had to break up with him again. No ultimatums, just me knowing I would rather go off and meet someone else who wanted what I wanted. It's a risk. And the risk is if he doesn't feel the same way as you, or isn't willing to go along with what you want, then you could be alone. So it really is something only you can decide to do.
Each time we broke up I joined a dating service. I never really wanted anyone as much as him, but I never really wanted anyone at all as much as I wanted to be a mum.
I have two kids, they bloody drive me mad, but I would not swap them for any man, ever, and I do really love my husband.
i am sorry to be blunt but if you want a child your time is running out, or close to it. Your boyfriend is playing with your future. Do not allow him to make this decision for you, acknowledge that if you stay with him you are choosing not to get married and not to have children.
Good luck. 