I half jokingly told my girlfriend that she should see a therapist because of her extreme fantasies.
Have you told her some of your fantasies?
I don't want to tell her.
Some people have some weird fantasies but it is not okay to make fun of them, especially if you won't even tell her yours. It will hurt her feelings and make her feel she can't share with you.
I just want to prove that she is psychologically damaged. How do you expect people to be honest if they can be ridiculed.
These things are just innocent thoughts in her head and nothing that she is acting out. You shouldn't make fun of her because she shared some personal things with you.