I tend to agree here, but there's a reasonable viewpoint that people shouldn't be able to consent to some things if they're assumed to be unreasonable or socially harmful.

For instance, we disallow the selling of organs. I'm all for bodily autonomy, but this seems reasonable.

At the very extreme case, one might say "I give you permission to murder me, if you provide for my family".

While I agree that self-direction and autonomy are essential rights (pretty strongly, in fact), everyone puts limits on them. For some people, the dynamics of prostitution might be that limit. It's not the case for me, but reasonable people might draw the line there.