Wow, I haven't been here in a minute.

Sarah (my SO, trans woman) and I have discussed kids in our lives. Before she started HRT, she told me she was unable to reproduce kids who were biologically hers because of physical issues. She has tried before with another partner and was unable to have a baby. Mix that, with HRT causing sterility after a while, our chances of having a child be biologically both ours, are none. She is open to us having a family through various means.
We have settled on, when we decide to start a family, us either going to a sperm bank or ask a really really good friend. Which case, I will have to get a lawyer to assure that our family stays our family. I'm fortunate to have some financial stability and we are able to do that. With other family's experiences with custody battles and such, I am protective.
In the end, it is call that both of you have to make. Though, with HRT, there might be irreversible physical changes which may cause permanent sterility, even if your partner goes off HRT. it is a good conversation to have with a doctor, get time frames of when things happen. Weigh your options, and see what will fit both your lives. I know it sounds like such a generic and unhelpful comment, but this a decision that will affect the both of you. I hope for the best.
OOPS: I used the word sterility instead of infertility. Yikes! Sorry folks.