Whether the couple is capable of having preferences probably depends on your definition of “preferences.” The more standard terminology for preferences by a group of people is “social choice function.” The main problem we run into is that social choice functions don’t behave like preferences. By Gibbard’s theorem, we can guarantee that any social choice function is either Pareto inefficient or unobservable (because it’s not incentive-compatible).
Sometimes, Pareto inefficiency is the price we must pay for people to volunteer information. (e.g. random dictatorship is Pareto-inefficient if we’re risk averse, but it encourages everyone to state their true preferences.) But I don’t see what information we’re getting here. Everyone’s preferences were already known ahead of time; there was no need to choose the inefficient option.
Whether the couple is capable of having preferences probably depends on your definition of “preferences.” The more standard terminology for preferences by a group of people is “social choice function.” The main problem we run into is that social choice functions don’t behave like preferences. By Gibbard’s theorem, we can guarantee that any social choice function is either Pareto inefficient or unobservable (because it’s not incentive-compatible).
Sometimes, Pareto inefficiency is the price we must pay for people to volunteer information. (e.g. random dictatorship is Pareto-inefficient if we’re risk averse, but it encourages everyone to state their true preferences.) But I don’t see what information we’re getting here. Everyone’s preferences were already known ahead of time; there was no need to choose the inefficient option.