I can answer this very directly, because it relates to my recent life experience.
I have a husband and a thing going on the side with another man, Sven.
My husband and I were trying to conceive when things got hot and heavy with Sven. I immediately started using birth control with both men for safety reasons, so procreative sex was put on hold. That went on for a few months, and then I stopped having sex with the Sven temporarily. At that point, I began having unprotected sex with my husband again. That way, there was no question of paternity, because on the cycle I conceived, I had sex only with one man. After I became pregnant, I resumed having sex with both men.
If your potential XX partner is poly, she'll probably be having sex with other men. But it probably won't be too much to ask for her to hold off for a few months while you try to conceive. In fact, you could do it like me: a few months of trying, a few months of break, and a few months of trying again. That way, she could even maintain her current partners, who probably won't mind taking just a month off here and there. If you and your potential XX partner have fertility problems, however, and it takes many months or years to conceive, this approach may cause strain on your poly arrangements eventually.
In fact, she need not even take the whole month off. I have irregular cycles, so it is not possible for me to pinpoint ovulation. That meant that I nixed outside sex for entire cycles at a time. But if your future XX has regular cycles and charts them, she will be able to pinpoint ovulation. She can have sex only with you until she ovulates, then give it a nice wide safety window of, say, a week after that. That still leaves her one week at the end of her cycle each month where it is not possible for her to become pregnant, and she could pursue outside sexual contact in that last week.