That is an issue with selecting "No" under settings / user settings under the Privacy for "Guest can view the site". The best option you have is to use this plugin, and set the privacy for "Guest can view the site" to "Yes". This will correct the issue where guests can open the "Contact Us" page. Just follow the simple instruction on the plugin.
P.S. If you use the plugin, and set privacy for guests to see the website, they can open other pages in the menu, so go to admin / settings / page settings, and edit each page you don't want guests to see by unchecking the box next to "Guests" on each of the pages that you don't want them to see.
https://developers.oxwall.com/store/item/1198