If you are interested in having children, or even entertaining the thought, you already know where John stands, so unless there is a possibility of changing his mind, then you know there's no opportunity there. You've even mentioned that it's not a 'big love' for either of you, which gives you a secondary reason to move on, at least for now. I say at least for now because if you got back together now, it's possible that a couple years from now, you and John may try again. It's hard to say that leaving what you already have for something that is unknown is the right thing to do, but you do know that you might want children, and you also know that it's not something he's interested in. I welcome your thoughts, let me know if you want to talk more
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Please do let me know if you want to talk more. I am at your service.