Your romantic partner sleeping with someone else is objectively not the worst form of betrayal - the fact it is universally treated as such is just a social standard that people blindly follow

Obviously different people want different things from relationships, and while I myself really do not care about my partner sleeping or making out with someone else (as far as they use protection and are careful), I do not doubt that some people find it important to be the only one who their partner touches in a sexual way.

For context: I am not polyromantic, I am not a swinger, I just do not care what my partner does with their body as far as they do not try to involve me (when I do not want to be involved), and as far as they are careful (because I of course do not want my partner to get hurt).

However, what I do doubt is that sexual exclusivity is genuinely the most basic must-have of a relationship for almost everyone. And that for those who genuinely find it important, it is always the most important one. What I do believe is that society placed a very high value on physical exclusivity and basically directly linked it to actual loyalty - to the point that I think it would be easier to say "we care about each other and are loyal to each other, let's go physically exclusive" than try to explain how I do care about their loyalty and do want to be loyal - I just do not think their loyalty is them having sex with me only.

Because I understand that in the minds of most people having sex with someone else = not caring about their partner.

But it makes absolutely no sense. There is no reason why someone cannot place most value on their partner only hugging them but not other people (but would not mind their partner having sex with someone if no hugs are involved), or on their partner only making tasty meals for them but nobody else, or on their partner never sharing any of their personal information online or with their friends, or on their partner never watching movies with anyone but them...

Again, I do believe that there are plenty of people who find exclusive sexual relationship the core of their romantic partnership - I just do not think it is the overwhelming majority.

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