Many proponents of homosexual marriage are perplexed, even offended at the idea that gay marriage creates a precedent or slippery slope that will lead to the legalization of other prohibited unions, especially polygamy. They see no connection between the two. I expect that most proponents of polygamy feel the same way about gay marriage, especially the Mormon fundamentalists who are not, to say the least, pro-gay.
Yet from the legal point of view, it would be hard to legalize one without legalizing the other. Thus far, the legal definition of marriage has been one man married to one woman. The "one and one" formula was nailed down in the 19th century polygamy cases. The "man and woman" standard has always been implicit in the law. Lately a number of state constitutions have been amended to make the man and woman standard explicit, as does the federal Defense of Marriage Act.
If, though, there were a successful legal challenge to either part of the "one man and one woman" standard, it would be hard to explain why the other part would not fall, as well. Restricting marriage to heterosexual couples was ruled a violation of constitutional equal protection standards in Massachusetts. There is no reason, in principle, why restricting marriage to couples (as opposed to larger groups) would not also violate equal protection standards. That did not happen in the Massachusetts Goodridge case only because no one asked the court to. If a heterosexual standard of marriage is just "prejudice" or "mere tradition" or "imposing religious standards," all those same arguments could, and would, be made about a monogamous standard of marriage.
To homosexuals, or polygamists, who want to get married, the two kinds of union are worlds apart. To judges and legal thinkers, on the other hand, they sure look like the same issue.