Are monogamy-only laws unconstitutional?

Apparently so, according to the Attorney General and the President. On Wednesday, they told us that they will no longer defend sec. 3 of the Defense of Marriage Act in federal court because it is unconstitutional, since it allegedly violates the 5th amendment and thus does not withstand heightened scrutiny. Here’s what sec. 3 states:

“In determining the meaning of any Act of Congress, or of any ruling, regulation, or interpretation of the various administrative bureaus and agencies of the United States, the word `marriage’ means only a legal union between one man and one woman as husband and wife, and the word `spouse’ refers only to a person of the opposite sex who is a husband or a wife.”

Although the motivation for this is to clear the way for same-sex marriage to become the law of the land by bypassing the state law-making bodies that the Constitution empowers to deal with such issues, there’s more to sec. 3 than the requirement of gender difference. There is a number limitation as well. So, if the entirety of sec. 3 violates the 5th amendment and thus does not withstand heightened scrutiny, it would seem to be as unconstitutional to limit marriage to two as it would be to limit it to opposite genders, since there are  “sexual minorities” who would welcome such a judicial discarding of capricious numberism. (The plight of the polyamorous bisexual comes immediately to mind). [Update: The following italicized section is a rewrite of the crossed-out portion above. The conditionals mixed with inference indicators confused some people]  After all, there are sexual minorities, the polyamorous, whose sexual orientation requires more than one partner. But if sexual orientation requires heightened scrutiny and polyamory is a sexual orientation, then the number limitation requires heightened scrutiny as well. So, if the entirety of sec. 3 violates the 5th amendment and thus does not withstand heightened scrutiny, it would seem to be as unconstitutional to limit marriage to two as it would be to limit it to opposite genders. I don’t know if the Obama people thought this through, but the implication of General Holder’s claim is not only that the constitution requires genderless marriage but it requires that a limitation of marriage to two must withstand heightened scrutiny as well.  This means that the 19th century federal statutes that made polygamy illegal in the territories are probably unconstitutional.