I don't think the post you're replying to is demeaning life choices. The grandparent is advocating for cheating and calling it ethical. The comment you're replying to is correctly stating that it's unethical to have an open relationship when your partner doesn't know it's open. Key quote from the grandparent:

> 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".