It is our own responsibility to manage risk tolerance. This morning I kissed one of my partners goodbye as she walked out the door to leave for her commute. My expectation is to eat dinner with her when she arrives home this evening. However, a small probability exists that she may fall victim to a fatal car crash. I am not sitting here sweating with anxiety because even though losing her would hugely impact my entire life, the probability of the loss is within my own manageable risk tolerance. If she were on her way right now to go climb mount Everest, go base jumping, or meeting up with friends to shoot heron; I may feel differently and may even be inclined to de-tangle or exit the relationship due to my own anxiety levels.
Ultimately the burden of managing risk tolerance falls on the husband. If the OP is saying, "no, I am not willing to family plan with my husband exclusively". That is a perfectly good reason to reconsider the relationship in total. Aligning philosophies in family planning should always be a major tenant of any type of relationship, even if that philosophy is "we wont have a family", people should be aligned in this regard...
The question about whether the husband's request is reasonable falls on the priorities of OP and boyfriend. If a poly couple were to say that they are using PIV abstinence for the purposes of family planning than I would consider it reasonable so long as they were in alignment on the objective and plan. Same thing if more partners are involved, whatever the prevention method is, if everyone is in agreement and aligned with the primary objective, its all good.... I understand that is not the case here...
What makes this situation a bit difficult is that to me, it actually sounds like neither OP or husband has much of a paternity preference... I interpret OPs position as having paternity Indifference, I do not interpret the OPs statement as her specifically wanting to have the boyfriends baby. It also sounds like the husband doesn't want to raise the boyfriends baby, "but probably would". In that regard he does not seem all that passionate about paternity himself, otherwise he would probably not be relying on the morals of a meta who presumably doesn't have any specific loyalty to him for family planning...
If I were planning a family I might be inclined to say my comfort level is abstaining from other sexual partners while trying to seed the womb... Children are big decision and large monetary investment... When I wanted to buy a house, I went without for a couple years to save for a down payment. Limited my enjoyment in spending money for a greater goal. I don't see any reason the same cant be applied to sex, the major component of that sort of cooperation comes from having aligned goals with a partner. And that doesn't seem to be the case here. It is as if the husband is trying to put a Band-Aid on his mismanaged risk tolerance, or as I have suspected from the beginning he isn't all that passionate about paternity in the first place... That should be hashed out. Particularly because if the OP is paternity indifferent, and the husband is not, why risk the possibility of having a resented child in the home when it could have been prevented by compromise over a short duration of time?
I will be bold enough to even suggest putting poly aside for a while, family plan accordingly and if incompatibilities do exist it is a good time to dissolve the the relationship before bringing children/more children into the world...