While it's certainly no fun to be a one port nation, and I think that they are acting in a selfish manner (there will be plenty of time to deal the the Danes after we beat the Dutch), I think it is within their right to do so. Until a diplomacy patch is released, we will just have to live with it. Lets all be civil about it.
Of course no system is without drawbacks! However, I do think that the current system, or lack of a system, has more serious drawbacks than a voting system. The only merit of the current state is that it's already implemented.
1. This is basically democracy, but there is nothing preventing you from requiring larger majorities than 51 % to form an alliance. It is obvious to me that a lot of people will be unhappy about the decisions that the majority make. But since most people have to deal with similar situations all the time, it should be manageable. Those who really don't like where their nation is going can always go pirate, like people did in the age of sail.
2. It would be easy to limit the number of alliances that a nation can have at any given time. Say 1, 2, or 3, whatever seems to work out. This would ensure that no PvE nations form, while still allowing for smaller nations to survive among the bigger ones. I actually think a limit on the number of alliances might be a good idea regardless of the mechanism to form alliances. Voting, mission/Jump-something-style, random (like decided by the king or parliament back in Europe/Washington), dev (kind of like random ), or whatever.