a post as being "read"?
You do not have to manually mark them.....
You open a thread and read the posts, it will mark them as all read for you.
When you come back the next time and it has a BLUE 'New' in the label, click the new and it will give you the next unread message.. All the other posts will be available to you is you want to go back..........
I hope that answers your question.
I right click them and open em in a new window, so I know where I'm at on the main index.