system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is bigamy illegal in most places?

I know that some religions allow men to have multiple wives, but why do most countries ban it? Why I can have multiple stable simultaneous relationships, but I can only marry one person at a time?
gpt
There's a combination of reasons. First, and perhaps most prominent among the reasons in the Western world is that mainstream Christianity (Catholicism and Protestantism) espouse doctrines of monogamous marriage. Many laws on this type of thing were written with explicitly religious ethics in mind (the late 1800s in the United States had a lot of this, including what eventually became Prohibition). On a more pragmatic side, there is a history of oppression among polygamous societies. While the relationship may not be by definition one that restricts the freedom of the multiple wives/husbands, the practical truth is that the two often go hand in hand. Also, from an interesting perspective, polygamy is really bad for non-powerful men. In a "normal" society there is a near match of men to women allowing nearly everyone to end up in a monogamous match. When many women (in all cases swap genders for polyandrous societies) can all marry one man, they will choose one with wealth and power who can support them and their offspring. This leaves an underclass of men who cannot hope to find a mate which, given biological drives, generally makes them unhappy.