I don't think there is a logical answer possible in this context. We're talking about messy humans. Reasonable is the best we can hope for.
Because I definitely think it's not logical to tell others not to insult oneself, but having no particular concern about whether a third party insults the second party. Nor do I think it reasonable. I don't expect anyone to be the moral police officer of every social group they happen to frequent. That would also be unreasonable. But to make an appeal on the basis of "people shouldn't be insulting to each other", yes, I do kind of think that that has the actually logical corollary that nobody should be insulting to anybody. I do think it is reasonable to ask a person making that argument, perhaps not so bluntly, "So insulting people is only a problem now?"