Without an article linked I don't know if this is addressed, but credit card processors have a whole service set up to prevent this. Merchants can perform a $1 transaction to verify that the card info is correct and that transaction gets refunded to the card within a day or so. This isn't considered a charge against the card.
This gets done whenever you check into a hotel or rent a car. I would be surprised if most porn sites (because let's face it, we are talking about porn sites) did not use this service.
Edited to add: And yes, it would be credit card fraud to do this, plus probably a violation of the CFAA in the hands of an up and coming prosecutor.Not to mention the fact that since the number space for credit card data is limited and there is a chance that you might happen to hit on a valid combination, in which case you would actually be committing fraud,
Edited again now that there is an article link: Generating card numbers is easy - you can find the formula online (and pick which card type and bank you want to impersonate!). You will still risk going to jail to use this, but I am pretty sure that you aren't going to make it through any signup process without a good expiration date and CVV.