Here's a hypothesis.
Monogamy is only ethical if both partners continue to choose it. Not just once, but every day; and without guilting each other into it. Each of the partners has to continue to choose it, and the choice has to be truly free; without conditions or attached strings.
Folks have begun to warm up to the idea that open relationships can sometimes work, for a few weird people. However, even among people accepting of this – even among those who are poly – the idea of physical loyalty remains sacrosanct. The idea remains dominant that, if you made monogamous vows, it is your duty to uphold them. No one respectable should cheat. Cheaters are literally worse than... racism.
Consider this, though.
Monogamy boils down to the expectation that you won't use your genitals in a way that isn't useful to, and approved by, your partner.
This is objectification. It is abrogation of each partner's individuality. It is dismissal of a person's independent sexual nature. It is a forced reduction of that nature to whatever might be acceptable to the other partner, and a dismissal of unmet needs that this forced reduction may create.
This is not love. Love is not forcing someone to shrink to a form in which they can't fully express themselves, based purely on your comfort and convenience.
Love is not something you give conditionally. That is trade. Love is given unconditionally. Except in jest, love does not involve statements such as: "I swear I'm going to cut off your X if I ever find you cheating!" That's not love, that's a threat of abuse. (Notice how it's only ever cute if it's said by a lovely woman?)
Many people live, and suffer, in non-consensual monogamy. This is monogamy to which a person once agreed, but might no longer agree to, if they could give it up without losing something important. Many of these are "dead bedroom" relationships; relationships that aren't even monogamous, as much as they are celibacy in a couple. Where one partner desires sex, and the other doesn't, so the sex happens once in a blue moon – and if it does, reluctantly.
This wouldn't have to be a problem, if the partner who doesn't want sex didn't expect the other to "just deal with it". They may have no interest in their partner's genitals – but they sure as hell expect no one else to touch them. If someone does – holy betrayal: may the vengeance of hell be upon thee!
I contend that this is objectification of the partner whose needs aren't being met. It's a dismissal of this person's independent sexual nature, and a reduction of their sexuality to a small fraction of what it naturally would be. Yet, people argue: "You made marriage vows – you better stick to it."
Well, no. If people have to stick to their agreements, it is a necessary stipulation that those agreements also be fair; they have to actually meet everyone's needs. Contrary to the broken moral compasses of the monogamous majority, a person cannot actually sign away their individuality with marriage.
We can make vows, and those vows have legitimacy as an expression of a couple's hopes and aspirations. However, marriage vows cannot be a contract. They cannot be a contract for the same reason that we would never, in this day and age, consider legitimate an agreement where a person becomes a slave of another; or where they become an indentured servant. Individuality is something you cannot give away. Not even with marriage.
The assumption of the monogamous majority, that their partner's genitals are theirs to own, is implicitly false. It cannot be true, because we cannot contract away our individuality.
Not infrequently, this false belief smashes headlong into reality, and survives this like a glass bottle crashing into rock. People realize that, despite their assumptions; despite their vows; they cannot actually own their partner. They never could; and this realization utterly destroys them.
Monogamy, in practice, can be beautiful. However, it cannot be beautiful to the extent that it's based on a false belief of owning a person. In order to work, monogamy has to be chosen; not by one partner, imposing it on the other, but by both. It has to be chosen not just once, but freely, every day. It has to not involve hostage-taking and coercion. There can't be any "You can't have sex, with me or anyone – or I'll make sure you never again see your children."
When monogamy is chosen by both partners, without strings; and continues to be chosen every day – such monogamy is beautiful, and healthy.
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