The polygamy argument is really a red herring. What ******* state allows polygamy? What laws are on the books concerning the rights of deciding who your next of kin is, in polygamous relationships? We're talking about a right granted by every state, to enter a legal contract, which is what marriage is. To deny that right to a specific class, without cause, is to deny equal protection.
What has "what state allows polygamy" got to do with it.
Your second sentence makes no sense.
If you deny consenting adults the rights to enter a marriage contract they find acceptable (and which has no issues with the laws of genetics), then how you can make your last claim.
If gays can marry, then I should be allowed to marry two women who want to marry me.
If I can't marry two women who want to marry me, then you've denied me a "right" and you (or the state) can deny gays the same right (to marry who they please).