Very good analysis and I think it makes sense.
I'm not running paysites, so I might not be right, but I think people are making chargebacks because they feel guilty to join a porn site, or their wife discovered it, hence your 81%.
Fraud must be eliminated and those people should face criminal charges.
Also, making people more open to porn, will help to avoid charbacks because of feeling guilty or because of the wife!
Cheers!