Why is polyamory so triggering to some people?

Is it because men are insecure that they won't be able to control their women as well? Or because women fear that it's hard enough to find a "normal" guy, so widespread practice will make it so common that they never will?

Like what are the underlying psychological fears that drive people (particularly conservatives) to be so hostile to what other people do in their bedrooms?
Why is polyamory so triggering to some people?
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