I was originally going to only add on my thoughts via tags but I think I'd rather organize it properly here, and yeah I ended up rambling a lot so bare with me. There's a lot of interesting points and perspectives in this thread and other reblogs and replies, but I feel there's also conflation of what's objective and what's subjective.
I have been in multiple polyamorous and monogamous relationships, and am now in a relationship that can't exactly be described as monogamous or exclusive, but definitely involves aspects of exclusivity. I'm hoping my range of experiences will help me be a little more objective. And just to be clear I am just trying to engage in the discussion, I'm not making statements on any user's character.
Firstly, I totally agree with criticizing that exclusivity is the default! But the reason for that is that every relationship is different, functions differently and under different rules. Assuming that relationships cant function without exclusivity is harmful, but isn't also assuming all relationships with exclusivity are inherently unhealthy? And the idea that OP is setting forth that exclusivity is unreasonable ("owning" and being "owed") and cheating can be justified is a bit unsettling, and I don't believe it's just because of my cultural bias.
Cheating isn't an "unfortunate side effect" of a non-monogamous person "trapped" in a monogamous relationship. That is someone knowing their partner has an expectation for them to be monogamous, and breaking that trust. And I think I can safely say trust is FOUNDATIONAL to at least most relationships, if not all.
Yes, it is not necessarily good that monogamy and exclusivity is assumed when entering a relationship, unless explicitly said otherwise- ideally, every relationship would have a full conversation about those two labels, and set those boundaries mutually and explicitly. And I totally recognize and respect that openly being polyamorous, brings about discrimination and even can be dangerous.
However, hiding that you are non-monogamous and engaging in non-monogamy is absolutely still harm (emotional harm is just as valid as "material" harm) to a partner who was expecting exclusive monogamy. If the non-monogamous partner is living their life healthiest when in multiple relationships, they should be communicating that to their monogamous partner. Whether that ends the relationship or not, at least the monogamous partner is not being actively deceived and breaking consent.
If exclusivity is a breach of your own boundaries and the freedoms you wish to keep, then you can tell your partners that exclusivity is not for you. And we absolutely should have better education about non-monogamy so that people KNOW they have the option to retain full control over their romantic and sexual endeavors.
But the implication I interpret from OP's post, which may have been unintended but is still what many people reading seem to have understood, is that non-monogamous peopple being 'unable to avoid' cheating is somehow more reasonable than the idea that some people are comfortable and happy handing over some control of their life to what would likely be a life-long living-together partner.
