Because everyone is known to be rational and because everyone therefore expects that 100% of everyone else will also vote for it, do you see how 100 people could rationally agree that they *WANT* to vote for the dam.

You agreed to the below.
'But if you can in fact see how enforcement of voluntarily agreed upon contracts benefits trade and can better humanity from a moral perspective, and if you can see that 100% of a particular group of people could vote for that; then we can proceed in seeing how a just society, utilizing democracy, could form.'

It was rational for 100 people to vote to enforce contracts. (Right?)
So it could theoretically be possible for 100 people to want to vote for a dam to be built (for rational reasons, as well).

And if all 100 farmers all agree to a vote on the issue, what is the objection? You have already agreed that 100 people could all agree to vote that they want to enforce contracts. What's the conceptual difference?