Earlier quoted context omitted.
They're not extremely high, they're the cost of doing business.
And the world would be a much better place if it was harder to impose higher costs of doing business on controversial, but legal businesses. Which is why I support solutions that undermine attempts by a couple monopolies to impose larger costs on legal, but controversial businesses.
That said, with $1.26 transaction fees + 1-2% to buy BTC vs. even 20% for a credit card in this high-risk MCC, the breakeven is ~$7, and while I'm not sure what adult content costs on a monthly basis, I'd imagine the fee difference isn't a whole lot.
It's also not an imposition, nobody is required to use credit cards. Adult content is largely and lucratively ad-supported. Some businesses choose to offer content on a subscription model like the NYT paywall, and some choose not to. I don't see anything inherently wrong with the status quo. The cost of doing business is being passed down. They accept credit cards because even at a 20% fee, it's in their interests to do so. If it wasn't they'd stop. Either way, they're recovering their fees from customers.