HTTPS is always required with Portal, whether you use LDAP/ActiveDirectory or not. Portal automatically comes with https enabled and no way to disable it. Many pages in Portal must be read over https (such as the 'My Organization' page) and links will break if you somehow disable https after the fact. To prevent problems the web adaptor can not even be configured until https has been enabled in the web server as well.
The thing that differs with ActiveDirectory/LDAP and web tier authentication is that HTTP (unencrypted communications) must be disabled and all communications must go through https. If you were using built in users you could have HTTP enabled.