Read the rules. They are very clear and don't ban discussion of polygamy at all. They ban promotion of homosexuality, but don't mention polygamy anywhere. Marriage is defined as heterosexual but not necessarily monogamous.
From the rules, it is clear they ban the promotion of adultery. From the discussion, it is clear they regard polygamy as adultery. Thus, a defense of polygamy is promotion of adultery, and against the rules.
Furthermore, they also ban the promotion of illegal activities, and thus polygamy can be banned to that reason separately.