I don't think I understand the issue with using 3D Secure, or Verified by VISA. It's something customers are already accustom when shopping online. You're buying a high risk item, from the perspective of the sell, similarly to downloadable content, or high price electronics. Solution like 3D Secure seems ideal in this case.
Companies like Stripe could easily say: Yes, we do accept payments for adult sites, but 3D Secure will be enabled on all purchases. I don't think that would scare of more customer than bouncing them between multiple payment provides and seeing the their payment reject multiple times.