There is nothing to stop porn sites from moving offshore. There are some porn sites that are hosted in the UK.

While a US citizen is bound by US laws, a foreign national is not. For example, a porn site could be hosted on the Bahnhof site (bahnhof.cloud). I did not see a customer limitation when it comes to porn at Bahnhof.

Someone hosting such a site could not travel or transit through the US, of course.

Extradition is only supported for things that both countries agree are crimes. If porn is not criminal in the foreign country, they would not extradite someone to Texas, for example.