There are no "default" open ports on users firewalls. Every game basically chooses its own port (and usually they try to NOT use ports used by others to avoid conflicts). I have not heard of any two games that purposesfully "share" a port that another game or application uses. For example if we default to port 80, every user running a web-server won't work. Now the likelyhood of two games running at the same time is next to 0, so I do see a good argument of using a common port for games, but as far as I know, no such "Default" exists.
Thanks