We have a small forum that runs on SMF software and we also restrict non members access.
However we have made it so guests' can see all the forums categories and titles (so they can see what they are missing out on by not being a member) but when they go to access a topic it prompts them to either sign up or log in. This method still protects the content but allows them to see whats available.
I noticed you guys have simply denied guests the option of even seeing what they are missing out on. Which is probably fine when your a massively famous forum as Myswag is, but for us, who is still growing rapidly the other option works better for us.
Just some useless info I guess