Thank you for your answer.
I found out, that registered users prefer to post topics if they are anonymous.
Usually they don't want to expose themselves with topics.
(strange, but they comment without problem as registered users. ...that's my strong experience from some forums)
So, instead to force user to log out and post as anonymous, would be great to have option - check box to click (if they want to post as anonymous they click) and then post.
This logout/login procedure is annoying, especialy if the user is premanently loged in.