Legalize consenting-adult-sex-work and remove the regulatory barriers that force these sites to use usurious, alternative payment processors.

If you must, tax these services to fund investigations and prosecution of abuse and trafficking. Note that the savings these services would enjoy from being able to use mainstream financial services would be so enormous that even with some kind of "sin tax" they'd still come out ahead.

(note, I generally don't approve of sin taxes, but since you asked, I think this is one option)