Multiple Counter-Strike: Source clients connecting to same server fails (SOLVED)
This issue really doesn't have anything to do with PFsense but it was a problem I faced for a while (I wasn't really sure how or what to search). Essentially, each client behind the same public IP must use a different client port if they want to connect to the same game. Otherwise when both players attempt to connect to the same server, the first one will connect and the second will fail. This applies to pretty much all Steam games from what I can tell (CS:S, Half-life 2, Team Fortress 2, Day of Defeat etc). See the following page for information on how to make this work:
This is just a post to help others who may be having this problem (Cybercafes etc?). I'm not sure if static ports are required for this to work or not. I have static ports ENABLED and it works for me.