This is a complex issue, but there is no general fairness requirement in contract law. There are some concepts that relate to fairness, such as consideration and unconscionableness, but they are not a fairness requirement, there are fairness requirements in specific narrow cases (usually in agency situations), but it would be wrong to say that in the general case a contract has to be fair to be enforced.
None of this is legal advice, just broad overview for the purposes of a purely academic discussion. There are a lot of details that are omitted, so do not rely on anything said here but consult your own attorney instead.