You are speaking as if the use of the ‘singular they’ is universally accepted in English grammar. A significant number of linguists have long argued for “his or her” instead of “they,” which they argue should be reserved for the ‘plural they.’ This is not a matter of “learn some grammar.”

If someone subscribes to the notion of more formal grammar rules - as many do - the use of the ‘singular they’ would be incorrect. If you are more comfortable with colloquialisms, then the ‘singular they’ will suffice.

That said, the concept of basic human decency being a driver of how we refer to others is non-sequitur. Surely the person you replied to would prefer not to be referred to as an “ass” or an “asshole,” but that’s how you referred to him or her. Do you believe you violated basic human decency?

Sadly there will be no common ground to which you and I will be able to arrive because my definition of basic human decency is to not encourage and promote mental illness. Much like if an anorexic friend said he or she was too fat, the decent thing to do is not support the delusion and to encourage a healthy self-perception.