"Yeah, whatever, feel free to die fighting against it"
I think advocating against somethig, generating a discussion, moving the Overton window is a valid, almost necessary thing. (I admit sometimes they threaten the things I like (or me), but most of the time, it's OK.)
Perhaps "agreeing" was a bad choice of word, instead discussing, debating, or just letting me know your opinion about the matter.
"Then don't do the thing you disagree with? It's a free and open web"
"Why don't you start with agreeing with my position first?"