(Suppose for example that a couple voluntarily sign a marriage contract stipulating death penalty, or even just flogging, for adultery. How can one oppose the enforcement of this contract without renouncing the libertarian principle?)

I want to talk about this. I do not feel that the contract described here is unacceptable. I find it strange to come to terms with how it might be the case. Would someone mind enlightening me about why they would feel it unethical to engage in a contract of the sort?

(By the way, I understand the practical objections to such a contract being allowed. People might sign it without fully realizing the implications of it, or they might be coerced by social pressures into signing it. Are reasons like these the only reasons? It seems though that people who argue that contracts like that should be banned are not generally saying so because of reasons like these, but rather they say it from some deeper moral principles, which I would like to know what they are.)