MC would not like it if I got pregnant by TGIB. I would not like it if I got pregnant by TGIB. TGIB would not like it if I got pregnant by TGIB.
We understand there is a risk, and if it happened, we would all deal, but we do as much as we can to minimize the risk.
It seems like you are putting the cart, or several carts, before the horse, and need to stop and see what they each think about the concept before you worry about compromise and balance. You may all already be on the same page, and your worry may be for nothing. Find out first. Then you can worry about limits and what is fair. (Though for me, I agree with you that I would not want MC to get another woman pregnant. TGIB already has kids with his ex, though, and I deal with that, so I imagine I'd deal with it if it happened.)
Pan Female, Hinge in a V between my mono (straight) husband, Monochrome and my poly (pan) partner, ThatGuyInBlack