I agree that sponsors are not councilors or therapists, they are broken humans doing their best to live the best they can in the confines of their experience. However, 12 step here is very hush hush on any outside issues that aren't conformed to traditional values. I am a non christian and a non monogamist.
If sponsorship is to work there must be some kind of relationship and to do that there must be some greater level of disclosure, trust and feedback. WE are whole people with whole problems. My sponsor dumped me because she felt my value system was not only wrong but indicative of some sexual addiction rather than a valid harmless lifestyle.
I loved her enough to give her a chance to try to explain herself in a less judgmental way and to really hear her rather than be defensive. She does have a right to feel and express her opinion and I did make an effort to really honestly look into her claims. I went to therapy, gave up my lover of many years, did a 4th step inventory, and when I found I was doing was fairly healthy I went back to my lover. That said, I have drawn much that is useful from the steps and traditions in all my relationships. There is much in the steps that enhances my spiritual practice but it is helpful to remember the steps and traditions are the ideal. Ultimately my relationship is to the steps and the higher power I subscribe to. Sponsors are not perfect or graduated and sometimes are not willing to cultivate tolerance and unity as part of their behavior. WE all get there when we get there. To focus on the feeling of marginalization and the resentment that brings me harms my recovery, so I need to find some other avenue of support for that part of my life or leave those outside issues out of the discourse with my sponsor.
I am sure there must be poly friendly sponsors and if it matters to you that your values match do the footwork.