The only way having multiple partners is ethical is if they are all aware of each other and it was agreed upon before hand that the relationship is open. What you're advocating for is extremely unethical.
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> partner upon discovering they are a practicing non-monogamist
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> ethical
Nah.
> partner upon discovering they are a practicing non-monogamist
You don't "discover you're a practicing non-monogamist", that's just a euphemism for finding out you're being cheated on. You either agree to non-monogamy or you don't but the only ethical time to give someone that choice is before you've started practicing it behind their back.
> I find demeaning others life choices publicly online to be anti-ethical, but what do I know. Please be nice.
It's actually the grandparent that keeps demeaning people's life choices by calling monogamy "boring", "vanilla" and "on the wrong side of history".
The problem is people attach too much ego to their relationships and jealously easily flares up when there is a hint that their partner may also be attracted to someone else. There are also financial motivations for catching your partner in an affair and immediately leaving them.
However, the fact remains that people already have these kinds of relationships, they are just not talked about or openly accepted. It’s a bit like being a gay man 40-50 years ago.
If it weren’t for society priming people to instantly dump a partner upon discovering they are a practicing non-monogamist, people might be forced to think for themselves and decide if that’s something their okay with, and I suspect many people might be.