I suppose that would depend on where the site is hosted. The hosting server is the site should not contravene the laws of the country where the site is hosted.
To be honest you would be best to visit a large adult website and find what it is that they do. I am sure it is common to have such disclaimers. I would just verbatim copy one.
They aren't required, but many webmasters believe that warning pages are a good faith attempt at keeping people from inadvertently seeing potentially offensive material.
Check with some countries and their laws against adult sites. I know that there was a law governing pornography and adult products sites to have this type of splash page for a site. There are heavy fines so better to check this out then get burned by some government. Hope this helps out
If you put a "Disclamer" on your site that people can see weather that be at the front (Spash Screen) or a txt link on your site you are covered by certain laws.
You don't need the disclaimer in the US. It is voluntary, as well as doing other things so that common web filters will block it. Parental filters, that is. The purpose of that warning is just for the above reason. It is part of RTA and ASACP guidelines. They hope all adult website owners will use it. In fact, if you do your adult website like they want, the normal web filters would filter the whole thing out and they would never even see the warning. There is no restriction in the US about such sites. Many people have tried. This is not related to sites that are banned, child porn for example. They also claim that many "unwary" adults stumble into these sites. This gives them a way out before viewing anything they might think is offensive. As far as other countries, China being a good example. Your chances of seeing an adult site are slim to none. The country filters stuff out. Probably the same for other countries who ban these sites. (Yes I know there are ways around it) Using it in the US just makes you a "good"(?) citizen. Stephen C