For the record (part 1) - my marriage vows did NOT include any nonsense about "forsaking all others" (I should know - I wrote them myself!

) We never promised
anyone that our marriage would be monogamous and anyone who thinks we did is making some pretty big assumptions (which I may or may not feel the need to correct. (Gods/Goddesses certainly didn't come into play as we are both agnostic/atheists.) We knew exactly what we were promising
to each other - which, in my mind, is all that really matters (the rest is just insurance and tax breaks).
I agree that historically in the U.S. "legal marriage" has implied monogamy, it also has implied heterosexual unions. The society/culture generally changes before the legal system does. I wholeheartedly agree with the statement that "legal" and "ethical" are not always the same. I don't give a flying fuck what the government thinks about marriage - as far as I'm concerned it is none of their business - they should stick to contract law and if any number of people want to register a domestic contract then that is covered.
For the record (part 2) - you may want to do a little more research...even here ("in the West") marriage laws vary a LOT geographically. In Pennsylvania, where we live, since 1973 "adultery" is no longer a crime (nor is "fornication"). Marital infidelity is a CIVIL matter that affects divorce proceedings (but "alienation of affection" is not..hmmm.) Bigamy ("legally marrying" more than one person) is a misdemeanor of the second degree and you can face up to two years in prison.
PS. If anyone is interested - PA is NOT a "community property" state and, as of 2005, abolished "common law" marriage (although will recognize common law marriages entered into prior the enactment of the law).