Henry and Alan both make interesting points which I think somewhat cancel each other out. Yes, we do have to distinguish moral from legal issues, but in contract law they overlap in a unique way that has no close analogue in other branches of the law. Contract law is unique in that the parties themselves decide the conditions that they must obey, and society at large enforces this.

Consequently society - other people - may choose not to enforce certain types of contract. Morally, why should they?

Thus, for instance, contracts 'in restraint of trade' are invalid under existing law. So are contracts intended to fulfil an illegal or an immoral purpose. In the past, the latter have included contracts for the purpose of prostitution, an exception which would obviously be illiberal. But, for instance, what about contract terms which benefit no one but do harm people who themselves have done no harm? Surely those terms are nothing but harmful. Why should society jump up and intervene by force?

A related issue is this: if no harm has been done, surely the plaintiff should not be allowed to sue for damages: there were none.