I just looked up a few porn sites and they don't provide Mastercard/Visa payment but they just take the national payment processor of the few (EU) countries I checked.
What does this mean? "national payment processor"?
Those are all nationwide networks that allow online payments (online in this case, but they also handle most non-cash payments offline) inside these countries. I think most countries have such a network (under various legal forms, for example CB is a consortium but Bancontact is for-profit, while I think Interac in Canada is a non-profit, etc) but the US doesn't have an equivalent as far as I know.