I was not sarcastic. I was entirely straightforward and sincere.

You were straightforward in the most mocking and least helpful way possible, maybe.

Earlier, you claimed your intention was to lend moral support to the OP against common_law's rudeness. But now, you are claiming sincerity and straightforwardness in your reply to common law that simply contradicted what he said. Those things don't fit together. People who are being straightforward don't make sincere comments to one person for the purpose of communicating something else to another. Nor do they make assertions without providing explanations for their reasoning process. Being vague and ambiguous about your ideas is the opposite of being straightforward, actually. A straightforward approach would have been to say that you thought his choice of language was inappropriate, or for you to advance right away the arguments against his view that you ended up making later on. Snarkiness is not sincerity, equivocation is not straightforwardness.

I am afraid your conversation practices make me unable to engage with you further

You were willing to engage with me after I said something "inexcusably obnoxious" and sarcastic, but you draw the line at a well reasoned collection of counterarguments? Pull the other one.