(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?)

Personally I'd be okay with the flogging version. Some people are into that sort of thing. As for death... legitimate governments generally consider murder a fairly serious crime, and refuse to enforce contracts which would require illegal activity of the signatories. I'm comfortable with having the libertarian principle superceded by criminal law. The interesting part of the question is: is there any choice a person should be allowed to make about their self or property, where they should not have the option of committing to a specific choice in advance as part of a contract?