I think the key point about contracts, which has been lost in a sea of extreme examples, is that a contract cannot be used to enforce lifelong involuntary servitude, or anything else that violates the rights of a participant. If you sign a contract agreeing to do something but then later change your mind, then the contract is intended to ensure that the other party does not suffer unduly as a result of that decision - not to ensure that you suffer to make them feel better. This means that you are contractually obliged to make it up to them as best you can, but that's all.

In the case of a doctor who wants to quit surgery, there is no way he can make it up to the patient if he lets them die, hence he is duty bound to ensure that they don't. That may mean having to finish the surgery, but he can probably get away with calling in a colleague in most cases.

In the case of a soldier who wishes to desert, he can do so but he must ensure that he does not endanger his fellow soldiers or the war effort in doing so. This is liberally and unfairly interpreted by the army to mean he cannot do so during wartime at all, but in this day and age the penalty for deserting in a way that does not endanger lives is likely to be minor.

A wife or husband who decides to leave a marriage cannot be forced by contract to stay, but they may be expected to pay money to compensate their partner for irreversible life choices they have made on the understanding that the marriage would last longer.

A person who agrees to work indefinitely as a slave, but later changes their mind can leave without owing anything since the other party has only gained by their generosity, and was never legally entitled to it. They may however be expected to help make arrangements for their replacement and give an adequate notice period, to avoid causing harm by their sudden departure. The exact same thing is true of paid employment, incidentally.

In the case of a software pirate, the contract can oblige them not to give away or sell copies of the software, and if they violate it they can have the software itself, and any ill-gotten gains confiscated. The contract cannot however impose an arbitrary fine of $500,000, or any other unreasonable penalty, any more than a prenup could dictate that a bride must submit herself to the electric chair if she ever decides to leave her husband. The penalty terms in a contract must be reasonable in order to be legally binding.