Still have the email confirmation? (You did use a real email address, right? Where they send the legally valid confirmation and information regarding the contract you just signed? Since, as was said, clicking accept is considered signing a contract.) And of course, they have full records of your IP address and email address used, and the terms on the signup page state that you must use a real address, so all they have to do is produce their copy of the email they sent to prove you should have seen the statement.
The email will explicitly state that after one day your card will be charged a recurring fee.
The exact wording from one site I just signed up for (not actually porn, probably not a site I should promote here though). "Your account will rebill automatically every month until you cancel it. "
Another: "Your suscription will automatically be renewed for your convenience until you cancel."
"You have been billed as CCBILL Ltd. for the amount of $24.95 for 30 days then $24.95 recurring every 30 days."
As for paying for it, it's more cost-effective than buying a dirty magazine. I memorize and process things too quickly, nothing is good more than once. And starting a site ripper one time is a heck of a lot easier than trolling the net looking around (once you find a good site of course). Still expensive though. It is a multi-billion-dollar industry after all.
Incidentally, most any place (of questionable moral value or not) will give you the benefit of the doubt once, so getting a credit isn't really hard the first time IF you are nice. As for telling them you didn't even use the service after the first day, they don't care, and they don't really have any way to prove it themselves. They are only a billing company, not the ones running the site. It's not their responsibility to figure out if you deserve a credit because you didn't use the service. Even if the site owners called up to confirm you didn't use it, CCBill might not care.
If your credit card company is going to fight this, then kudos for you having a good creditor, but you are still in the wrong here.
Oh yes, you could also have contacted the site itself. It's not likely, but slightly possible they'd have issued a credit themselves. However they'd have been losing out on whatever percentage CCBill retained, since CCBill is only going to pay them X percentage of that 39.95 but they'd be giving the entire amount back to you.