Bill: That means that they view these voluntary choices of others as oppression.

That’s correct, the thick conception of libertarianism they (I) defend includes an explicit claim that voluntary social interactions can be oppressive without ceasing to be voluntary. I don’t see this as a problem for the view. I see it as the point of the view.

Bill: Unless they assume some massive amount of masochism on the part of those they’re claiming are oppressed, they’ve given themselves veto power over legitimacy of the voluntary interactions of those who don’t adopt similar attitudes …

I don’t know what kind of “veto power” you have in mind. If you think that a position against non-aggressive forms of oppression involves some tendency to individually disapprove of, or, say, to non-violently socially stigmatize, at least some behaviors, interactions or relationships which are admittedly voluntary (in the sense of being acted out through strictly non-aggressive means), then of course you are right about that. But is that “veto power” in any meaningful sense of the term? Certainly not in any meaning of the term that involves an exercise of aggressive power. Again, the point of the view, as stated by those who advance it, is to provide some considerations in favor of addressing non-violent social problems through non-violent social means. (Rather than, say, holding that there just are no non-violent social problems, by definition.) If, on the other hand, you think that calling X “oppressive” means assuming “veto power” over X in the sense of legitimating the use of political force to repress X, then, again, that is what the people in question (me included) explicitly deny, and have denied all along. (See for example this discussion of “the authoritarian theory of politics.”)

Given that we have explicitly called for the use of non-violent social activism and cultural change instead of any use of political force to address the oppressive-but-non-aggressive dynamics that we’ve discussed, any charge that what we’re proposing must ultimately logically lead to criminalizing those dynamics seems to be completely unfounded in what we’ve argued. Of course, you might hold that social stigma, cultural politics, etc. can all be problems even if they are not leading into political force — you might hold that the attitudes we take are, say, narrow-minded, or intolerant, or busybodying. And I don’t agree that they are, but you might have an argument for that. (Certainly I don’t think that “social stigma,” say, is something harmless when misused, even though it may be non-aggressive.) But then of course you too are advocating a thick conception of libertarianism — just one which connects libertarianism, say, with a very broad norm of social tolerance, and which exercises a great deal of skepticism towards any sort of social pressure or conscious non-violent activism, rather than with the sort of anti-oppression activism that I have tended to promote. And the argument is not going to be an argument about whether libertarianism should be conceived thinly (as a demand strictly independent of other social or individual norms) or thickly (as integrated with other, interlocking commitments) — it will just be an argument about what other commitments a thick conception of libertarianism ought to be integrated with.

. . . The "mutual aid" societies are intended to help you pay for the appendix to get taken out. Not to help you out with direct service provision. (At least, that's certainly how self-described mutual aid societies and friendly societies operated when they were relatively common, in the late 19th/early 20th century -- as a form of grassroots, consumer-owned insurance.) For that matter, the welfare office doesn't cut out your appendix either; they give you money which you pay to the doctor who does the cutting.

“If one takes the notion that the traditional social customs voluntarily practiced and accepted represent some sort of “coercion” on par with the government’s use of force, an aggression, ….”

If one took that position, maybe they would have the problems you describe. But that is explicitly not the position that is being advanced by work on “thick” conceptions of libertarianism. The entire point of a “thick” conception of libertarianism, as explicitly defined in articles like this one or this one, is that libertarianism may appropriately concern itself with feminist, “countercultural” and other anti-authoritarian concerns even when those concerns have no direct connection with acts of coercion and even when those concerns do not involve “an aggression,” because (they argue) libertarians as individuals, and libertarianism as a political theory, may appropriately concern themselves with at least some things that are non-aggressive but oppressive for other reasons. (So a “thick” conception of libertarianism holds that non-aggression is the only social norm that is enforceable. But that it’s not the only social norm. And it’s not the only social norm that is significant to libertarianism.)

Maybe that claim is wrong, too. But whether it’s wrong or not, it has nothing to do with the different claim you are indicting here.