I don’t know why everyone brings up telephones or electric grids. Your ability to purchase porn with credit card is not a right. It’s hilarious to even think about. Also given the stance in some countries like the US or parts of Australia it’s a hard argument to make. It’s far easier for these companies to self regulate for matters where there are constantly changing social and government norms.

Nothing is stopping you from buying porn but it’s quite possible Visa does not want their brand tied with more niche content.