Earlier quoted context omitted.
It seems like it's pretty easy to comply. Pornhub and others don't have any problems complying with TX.
According to this pornhub is blocked in 25 states including Texas. https://mashable.com/article/pornhub-blocked-states-2025
Pornhub itself is doing the blocking; it uses geolocation and denies services to IP addresses from jurisdictions with age verification laws. The laws are usually not structured so as to require a third party such as an ISP to block noncompliant sites; instead, the governments of the states with those laws can sue the porn sites and their service providers (Verisign in the case of .com domains).