I guess I don't understand how arguing women shouldn't be treated as objects to be bought and sold is a conservative position.

That isn't the position though. The position is, "prostitution should be illegal", which is the norm. Thus, defending it, no matter the reasoning, is the conservative position.

This being said, every argument can be framed as a positive or negative position. You're taking the positive position of, "women should not be able to bought and sold" but, "women should not have the right to sell sexual services" is just a negative interpretation of the same argument. The latter sounds much more conservative than the former.

More philosophically speaking, it's tough to argue why sex, specifically, should be a prohibited service to sell while other services are not. You sort of have to turn to external, conservative, moral values to differentiate between them. I'm sympathetic to the argument that sex work can put someone in a position where they are demeaned in the course of the transaction, but I don't think that is universally true, and I think we could point to other jobs where this is the case as well.

While we're on that, it strikes me that we don't actually give people that much freedom to decide what to do with their own body in matters of health.

I just sort of disagree empirically. Though you can cite some examples of restrictions on medical decisions, they are relatively rare. You can make PLENTY of decisions that can be quite harmful if you choose. I would also sort of argue that many of your examples aren't really medical in any true sense of the word. Choosing to sell an organ or chop off a limb arbitrarily aren't truly medical procedures, they aren't treating anything. I think it needs to involve more than circumstantial biology to be a "medical" decision. But I don't want to tread too far into just discussing semantics.