There were some rumblings around the blogsphere a while ago that you could host Adult material inside a free Google App engine account. No idea if that is still true though, even if it is then it would be nightmare to build and maintain.
Of course finding hosting is only the first hurdle to cross, you then have to comply with the regulations under Title 18, Section 2257 of US law. These basically state you have to keep records (and be able to show them on request) that prove the model was over 18 at the time of the shoot and agreed for the pictures to be used commercially. How old you are as the site owner is actually of little importance, kids under 18 own businesses all over the world.
So if you try operating a porn site where the server is on US soil without that paperwork, charges of copyright infringement will be the least of your worries. If the content is being generated by users say for instance a Naughty Singles forum then the age verification is going to involve storing credit card numbers and that opens up further regulations for data security & privacy.
Plus your support people are going to have to be dam quick to remove material that looks suspect or runs foul of obscenity laws because as owner of the site, you are responsible for what goes on and get arrested first while law enforcement goes over your access logs to get the uploader.