Ok... (trying to get past some weird browser hijacking). Anyway, I forgot that there IS a 'mark all read' button, but it's not easy to find, obviously. Since I have to read every post, I never use it. That's my excuse for forgetting. 
Go to the bottom of any thread list within a forum and there will be a "more options" button (or something like that) on the left side. Click on that and you'll find a "mark all read" button at the bottom right of that page.