Start your site with a warning page without pictorials alerting your visitors that it contains adult contents and to allow them to consent to the disclaimer and enter or to exit from the site.
i nice warning page that state that the follwoing content is suitable for people aged 18 and over (21 in some countries/states), and then have 2 links one thats say i am over 18 (going to the main site) one thats say im under 18 (that goes back to google or where ever)
To avoid getting into legal issues, it is recommended that you put a legal note on the very first page of the site which allows access to the site ONLY if the visitor agrees that he/she is above 18 yrs of age.