"...yeah, that's the defensiveness" appears to discredit them based on the mindset of the arguer rather than the merit of the argument.

On reflection, I think this statement specifically is my problem, and not because of what it's saying about the argument, but about the arguer. My reaction is something like "well, damn, now if I object I'll appear to be an unnecessarily defensive jerk, even if I'm right."

It feels like "God will send you to hell if you question his existence"; where that one exacts penalties for the act of figuring out if there really are penalties, yours socially censures the act of questioning the justification of censure. Such double binds always strike me as intellectually dishonest.

Again, I don't think you actually meant it that way; it just pattern matched on certain similar arguments (which I'll leave unstated to avoid a mindkiller subthread) by people who actually do mean it that way.