You don't. It's optional for guests. It's there so those who would like to contact you "off list" are able to do so.

If you would rather not disclose your address to the world, register and become a member. Not only will you control who has access to your e-mail address, you can also privately converse with other members through the member area without disclosing any unintended information about yourself to anyone.  See   (You need to Login or Register later in the help section.
   
Exchange only the information about yourself you wish to make public! Become a member.    
   
Your privacy is sacred. QuietJourney will never share any member's e-mail address with anyone for any reason. (Well... unless required to by law. And even then only kicking and screaming...)