A contract seems like a dumb idea to me. It simply won't work. If two people are attracted to each other, then they will do as they please, contract or no contract.

Where it can get tricky is where one person's attentions are unwelcome by the other, or when an attraction between two people goes wrong somehow. In those situations things can cross over into claims of harassment, and it can all get very messy. I've seen it happen.