Most other people in my society would help enforce a contract in which a young person is drafted to serve in the military or taxed to give money to the elderly without his consent. A libertarian would not - a moral judgment.

It is not my impression that the draft or taxation operate on the basis of contract. ("Social contract" is a fiction — we're talking here about actual contracts, entered into by two contracting parties, and enforced by a third; as in the example of marriages in the original post.)

Your claim that the "freedom of contract" view is "amoral" ignores all similar moral decisions.

Sorry, I think we are using "amoral" to mean different things. It was not my intention to say "libertarians are amoral" but rather that the absolute-freedom-of-contract position demands that the enforcing party not be moved by any objections to any possible contract terms. I thought this was perfectly clear; apparently not.

In order for parties A and B to have "absolute freedom of contract", the enforcing party C (e.g. the state — or a cooperative, the neighbors, a Nozickian private agency, or what-have-you) must ask no questions beyond "Is this really a contract between A and B?" If so, enforce it; if not, don't enforce. If C is ever moved by objections to contract terms and ever declines to enforce, then A and B do not have absolute freedom of contract with respect to C's enforcement.

Rich men can afford to enforce their own contracts

Only in extreme cases.