There is a problem here, in that if you don't use the Membership classes in some form then you have to check and enforce the access rules manually in each and every page. If you use Membership, then it is all handled for you on a folder-by-folder basis via the web.config settings. If you don't, then the configuration is ignored, and you have to do it yourself - this can lead to mistakes and security holes.
Have you considered implementing your own custom Membership provider? It's not complicated, and it does get rid of a lot of the hassles with a brew-your-own-security system.
Have a look at these:
MSDN: Implementing a Membership Provider
Custom MembershipProvider and RoleProvider Implementations that use Web Services