Actually, I'm going to be really blunt.
Warning: This is not true of all childless relationships, nor of all people who don't want children. I'm saying that now, to be really explicit. I don't want people saying, 'That's an outrageous thing to say about people who don't want children.'
Yes. It would be an outrageous thing to say. And wrong - ethically and factually.
And that's why it's not what I'm saying.
So, OP, reading between the lines:
You conflate masses of things here: selfishness, not wanting children (him), wanting children (you), mixing up care and control (they are different, you know. Similar when caring for an infant human - but different: and very different when we talk about adult humans).
I think your misgivings about this relationship extend further than the have-not have children issue.
Instead of suppressing that intuition - burying it under the 'have/not have children' issue - you need to face them square on.
Issue 1. The biggie.
You tell us you want children.
It sounds to me as though what you enjoy in this relationship is 'playing mum' to your adult male partner.
In return for 'caring' for him, you get a lot of control.
And you get to play out your maternal fantasies in a low-risk setting (after all, he's not really a child/baby, so any deep fears you have about not coping as a mother/failing a real child, need not be addressed - he's an adult and not, in actual fact, dependent on you).
He likes this too.
He likes being cared for. And in return for the control he cedes to you in your caring, you give him control in 'adult' aspects of your life: eg. Over dates.
You don't say it but I'll bet you give him control sexually too.
So, he gets his fantasy of a mother-he-can-fuck.
You get your fantasy of a-child-that-isn't-overwhelming.
And you both get to ignore this fantasy because you also play-act adult roles of dominance and adult sexuality.
Everything is therefore able to tick along.
Obviously, the entry of a real, actual child into this scenario is going to blow the complex fantasy to smithereens.
For both of you.
I think you know this, deep down.
Which is why you're posting on Mumsnet, rather than having an adult chat with the person you're considering raising a child with.
- Issue 2.
Your self-esteem.
I honestly think you need someone - or someone's - in your life to help you believe how worthwhile you are.
I really hope I haven't been too blunt.
And, of course, I could be wide of the mark.
It just seems to me that you are really confused - and you are posting here precisely because you are hoping someone will help you with a question you don't quite know how to frame.
The great virtue of an on-line, anonymous forum us that you are free to read people's suggestions and thoroughlly discard thdm.
So, you can read this and think, 'That I'd so wise of the mark.'
And that's good.
Because then you know that is one thing that can be dismissed - and it will help you get closer to the real thing that's bugging you - and thereby find your solution.
So / feel free to dismiss this.
It's only a suggestion.
Good luck.