Bingo.
You can have that case. I want to find a person who claims to be a polygamist who wants benefits for two partners. That would be a stronger case.
Yep, if the criteria is benefits for those who cannot legally marry, then they have to accept polygamist domestic partners, underage domestic partners, animal domestic partners, 1st cousin domestic partners, and any other partner. It is also most likely illegal to exclude those who choose not to get married. It's their right to choose and not be discriminated against.