If the people involved have families, someone else could very easily be harmed. If the voluntary interaction involves polluting a river other people drink out of, someone else could very easily be harmed. Continue ad infinitum. If you think a consensual interaction between two people isn't capable of harming anyone else, you haven't thought it through.
Your theoretical construction always makes two presumptions: 1) the parties have equal information, 2) the parties have equal capacity to contract. Neither is typically true. An interaction may be consensual and may not cause harm for years or even decades later; and one party may clearly understand the terms he designed while the other party does not.
The intervention is not unwanted. Like I said, we got legislation--all your rules you're complaining about--because the people demanded them, when consensual interactions failed. People demanded rules recognizing that one party to a bargain tends to have better information than another party, and that not everyone has the same capacity to understand the agreement.
