It seems like a no brainer to me. If gay marriage is constitutionally mandated, then polygamy is really just the merest corollary to that.
Cases of polygamy were not sanctioned by God. They are in the Bible as examples of sinful ways of life; not even Jacob got away with it without being burned. Abraham was monogamous apart from his tryst with Hagar. Isaac was never mentioned as being polygamous either, but monogamous with Rebekah. Esau was a polygamist. David and Solomon were especially plagued because of their polygamy.
The first example of a polygamist in the Bible is the first man to be named Lamech, a sixth-generation descendant of Cain’s line (from Genesis 4), and he is mentioned as being a shameless murderer to boot.
Agreed. Polygamy at least has a long history of acceptance, while homosexuality has almost always been viewed as a disgusting perversion of nature.
The Supreme Court banned polygamy in a decision in the 1800s, that basically said ‘This is a Christian nation and polygamy is awful’. However, the Supreme Court often reverses itself, and will have no rational basis for rejecting polygamy if gay sex turns into gay marriage.
Fundamentally, there is one aspect of marriage which has been essentially universal throughout history among nearly all societies which continued to thrive after the death of their original founding members: every woman is recognized as either being unmarried, or having exactly one husband, and in the latter situation shall be forbidden from having sexual relations with anyone but the husband without the husband's consent (which he may very well never give). Every marriage must thus involve exactly one man and at least one woman. A union of two men would involve two men and zero women; a union of two women would be slightly less abnormal, and might conform to some societies' norms if the two women brought in a husband.