support polygamy as a regular institution society needs to align itself to suppress the rights of women and younger men
Society doesn' t have to support polygamy... It would just have to make it legal... and it wouldn't require supressing anyones rights... The only possible supression I could think of is that hospital visitation would require a major overhaul to allow for multiple spouses. Polygamy is simply having multiple husbands/wives. The issues that you speak of have to do with certain cultures that condone polygamy. On the other hand, there is a minority of people that are forced not to marry because of their multiple consenting partners.
Of course, gay marriage is a more immediately pressing issue when you're talking about minorities not being allowed to marry.