There are problems I have with polygamy on a logic level.
If you are a polygamist, naturally you have multiple partners. But does that mean only one person is entitled to multiple other partners, therefore asserting who has power in the relationship? And how many multiples of partners does it take before you're simply sleeping around with people with little-to-no relationship value for most of those partners?
The points of having one partner are:
A) More time together meaning more time to raise kids, instead of having many kids from all different partners to deal with
B) A symbol that you're willing to give up your loveual/romantic freedom for the sake of one person
C) To allow for more genetic diversity, since allowing one person to share their genes with many partners reduces the amount of variation within the next generation (not a problem in small scale, but if we all became polygamists then it might be an issue)
Personally, I don't have an issue with it, but I don't really see the point, and I'm more interested in a long term relationship with a single partner over trying to get as many as possible.
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On the subject of Marriage:
I dunno why it was created, but I know why it (should) still exist.
It's supposed to be a celebration of the fact that you're forgoing your entire right to forget whoever you want and do whatever you want (like an animal) to instead be and share life with somebody you care for deeply. The traditional ways of marriage are boring, but its role is to remind people that marriage is pretty loving sacred on a honour level.
I don't have a problem with divorce, but I personally don't want to do it. Going back on marriage vows feels like a giant waste of time, effort and basically saying that you can't trust my own words after I broke the vows I made. Seems silly to me. That's why I find people who rush into marriage and break-up later to be totally insane. Marriage isn't just a party for people who like each other, it's a little bit heavier than that.