Americans in general seem to agree that polygamy is a bad thing. My question is: why do we think that way?
I think there are several reasons why some (fewer and fewer) Americans think polygamy is a bad thing.
For one thing, some elements of American culture have relentlessly insisted that monogamy is the only right way. This is drilled into everyone from an early age. This comes from those who want to control the sexuality of others, and perhaps those who wanted to protect women back when they were considered property, couldn't vote, and had little access to education and the professional world. Today, we have much more gender equality. But most people, including those touting monogamy as the only right way, do not actually practice lifelong monogamy. Even most people who get married only once to one person have had other sexual partners prior to the marriage. Then there’s divorce and remarriage, cheating while married, single-encounter or temporary threesomes, swinging, swapping, and more. People who have done these things may still say that polygamy is wrong, or even “monogamy is the only right way.” But their actions speak louder than their words.
Another aspect, I think, is a connection in the minds of some to male dominance, Islam, or mysterious, isolated religious groups in North America where a middle-aged religious leader marries underage girls.
Anti-Mormon feelings in nineteenth century America probably solidified opposition to this freedom to marry as one more way to attack Mormons, as the largest Mormon group practiced it at the time (but no longer.) It is ironic that so many Mormons have recently opposed the same-sex freedom to marry, given how their ancestors were treated.
When people really think through it in a calm way, without invoking their fears of other religions, they really can’t come up with a good reason why consenting adults suited to polygamy should be denied their freedom to marry.