On one hand I understand your worry. I'm married, hubby and I have kids, we've both discussed it and neither of us want more kids so have taken care of that issue on our ends. That means, my boyfriend one day will want kids, probably a wife. I can't be that for him, so does that mean that at some point this relationship will end?
I agree with Marcus here that you are putting the cart before the horse and setting yourself up for pain. Relationships end, many times you are dating someone and it ends. Amicably or not. So? You stop dating? Forever??
Each relationship is it's own relationship. Meaning if you subscribe to the idea of poly you understand that a relationship is not successful or fulfilling solely on the basis of ending in marriage and kids! It's successful if it's loving, if it works for the people involved.
My current boyfriend and I have discussed this a lot, and the discussion has changed over time. At first, we sort of tried to accept that at some point, he may have to say goodbye to me. That if he meets a woman he is wanting to marry and have kids with, she's going to want him all to herself. Then it evolved. He's happy with this model and while we both understand that especially with NRE and all that he may back off a bit, spend a little less time with me while they are building their relationship, why does it mean that I CAN'T remain in his life?
Now a days we have discussions on how he hopes any future relationship and I get along and how I plan on spoiling their children just because I can!
The point is, why go into it assuming it will fail? Maybe you will break up maybe you won't, but why is that dependent on the fact that you are already married? If someone is dating you and knows you are married they should be pretty aware that the relationship model you two will have will not include a legal marriage and be okay with it! Lots of successful relationships have nothing to do with marriage!
Me: Late 30s pansexual poly.
DH: My husband of 19 yrs and father of 3 teen girls.
DC: LDR of +4 year