By default every website including the adult websites can be accessed using the Safari on iPhone. But you can block specific websites on iPhone, iPad and iPod Touch device running iOS 7 or later, by doing as follows:
1. Open the Settings app on your iPhone and go to the General settings
2. Scroll down to the Restrictions and tap to open it
3. Make sure the Restrictions is enabled. (You either need to enter your restrictions passcode or define a new passcode to access the Restrictions settings)
4. Scroll down to the list of Allowed Content and tap the Websites option
5. Tap to open the Limit Adult Content option and then under the list of Never Allowed websites, add the website you want to restrict the access to.