The only solution to multiplayer problem
The port problem isn't new, I have had the same problem on systems like battle.net in the past. However, these days it's becoming less and less common because a lot of developers now have dedicated multiplayer hosting servers for their games (almost any FPS online game). To survive, I think Sins needs a similar solution, with Ironclad providing a bunch of preprepared "hosted games", where one player can join and become the administrator of the game and change settings, and others can join.
That way, the port problem lies with ironclad's servers, so the vast majority of people wouldn't have to deal with it; and if a tech savvy person wanted to host their game with a port forwarded computer, they still could. It can't require too much resources on Ironclad's part, and saving the multiplayer will keep the game profitable.