> You totally have the legal obligation to provide for your children, if you have them, and that almost always involves using your body in some way

You are not legally obligated to donate your organs to your children, fwiw.

I think the discussion of ethical and legal obligation cannot possible be given broad swath of "no one is obligated to anyone" nor "you are entirely obligated to someone in every way". This isn't how reality works. Parents are considered obligated to their children, unless they choose not to parent their children and withdraw their parental responsibilities under existent legal frameworks. At the same time, parents are not entirely obligated to their children, as parents are not fined or jailed for not donating their blood or organs.