Well.. it is not that simple. In most of the jurisdictions, the authorities want the porn sites to confirm the age of their customers, to make sure that individuals who are younger than 18 years of age are not allowed to access adult content. If you are making the payment with a credit card, then there is no need for additional verification. However, if you attempt to make the payment using BTC, then you will be asked for some additional documentation to prove that you are above 18.