It's actually an example of the market mechanism solving the problem, you're just ignoring a few actors here: The payment processor of Patreon has an interest

It's actually an example of the market mechanism solving the problem, you're just ignoring a few actors here: The payment processor of Patreon has an interest not to deal with the porn, therefore declines the business. The payment processor of competing actual porn platforms has a market niche to take exactly that business. All the middlemen that are required here (for complex reasons) get paid for making it happen. Nothing is lost, therefore no inefficiency, therefore no failure. The money not spent on one performer through Patreon can instead be spent on any other performer wise enough not to rely on Patreon for payment.