Social contract theory is a broad ethical paradigm which focuses on the moral value of agreements between individuals or categories of individuals. The general premise is that it is morally acceptable for people do whatever it is they have agreed to do.
However, I think our actual experience is that there are some things that could not reasonably be contracted for. One example is the story of a man who agreed to let someone else kill and eat him. Did the "killer" do anything wrong, since he had the consent of the person being killed?
This leads to a more general question - what are the moral limits to agreements? Answering this question should rely on works by notable social contract theorists, so the question is What have notable political and social theorists identified as limitations to agreements?