So even though my router and firewall are configured for sins, it still claims that most people won't be able to join my game.  What gives.

Here's the story...

Port triggering 6112 to 6112

port forwarding 6112 to 6112 for this address (which is static)

makes no difference if DMZ is enabled or disabled, I still get the same messages.

Aside from simply plugging my computer straight into my modem (instead of router), how can I get the message to go away?

I've had 2on2 games fill up within 5 seconds, so there isn't a problem except for the annoying message.