I'm not sure I understand -- you'd be ok with her having your husband's biological child, but only if there's an even chance that she'll be carrying your child and not hers? I don't say this to be cruel, I swear, and I apologize if it comes off that way, but how is this not a jealousy issue if all your concerns go out the window as long as you can be equally involved? When the end result might be *exactly* the same as if she and your husband had a child together the traditional way? It seems, then, as if the end result of your husband having a child outside of marriage isn't the problem for you, it's simply whether or not you get to be a part of the biological process... that seems very controlling to me, and quite unfair to her since you might be taking away her opportunity to have a child who is biologically related to her.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.