You don't mention anywhere in your post a history of being attracted to women. More specifically it sounds like you're not attracted to THIS woman (for a variety of reasons). No one should EVER be sexually involved with another person for the gratification of someone else. How does your husband feel about you exploring poly to find a partner who you are into? Is it only ok with him if you have a shared partner who he finds and brings home to you? What if you were to meet another man you fell for? Would he be on board with you exploring that?
Also why do YOU need to be poly for him to be? If you're not feeling it and don't desire another partner but support him in his relationship where's the problem? It sounds like you and he have a lot more talking to do.
Everything will be ok in the end. If it's not ok it's not the end.