She doesn't want any more kids
I am in love with a young woman. She has two children. She had them before we met.
I do not have any children as yet. She told me that she wants us to get married, but she said she doesn't want any more children because she had a hard time when she was pregnant with her second child.
I would want my own. I can be a father to her kids, but I cannot call them my own. What do you think?
If your desire is to have children of your own, you should let this woman know how you feel about that.
Tell her that you cannot marry her because she does not wish to have more children. Tell her you respect her position, but you cannot marry her, so you will have to move on.
Don't hang around at all, my friend. Perhaps she might have had a tubal ligation to prevent her from getting pregnant, so wish her well and end the relationship.