You have to link a valid email address to your forum account mate. But it's not necessary when registering, you can just make up something and add @gmail.com and later on decide to change it to a valid one. The reason be that if you loss access to your account, you will need the email linked to your account to help you recover it and change to a new password.
If you do use a personal email address and don't want it to be public, you can turn it off on your forum account settings. Its there on your profile..
Thanks for your nice opinion share with us. I myself created an account with a temporary email and later created a good email and set it up from the settings. It was great that when changing, the email could be changed very easily.