What an interesting question.
It all depends on how you expect the users to use your site:
If you want users to visit your adult site for a minute just to get one or two things, then at the very least, have the users read an agreement, then verify they read it by having them answer a skill-testing question that adults can only understand and then if successful, make the server send a cookie to the client browser so the client is then authorized to access the site.
If you expect users to do more involved actions on your site (like adult item shopping as opposed to looking at one and only one picture), then you could require users to create accounts then make them verify their email address and require them to send a recent photo of themselves.
Point is, just make sure users are verified as adults and when they are, the server can send a special cookie to the user's browsers to identify them as adults.
Having all html adult pages public including the index html page as a warning page isn't good enough because if an adult enters the site and is greeted with the index page and decides to declare himself as an adult, he is then taken to the next page on the site. He could tell a kid the new URL and the kid could type it in and BAM! kid sees adult content.
You will need to learn some kind of programming language such as PHP if you want to be able to run this kind of site successfully.
And whatever you do, don't think about getting the adult portions of the site indexed with google or any search engine unless you make the pages family-friendly enough for google.