You appear to be stating that she charged other items to the card. If that's the case, aside from it being unrealistic in the first place, there's no chance that somebody else's credit card somehow popped into the system. It's the number she was repeatedly using. The idea that many people were using the card and sending items to you and your girlfriend? Come on.
The police have circumstantial evidence that you were involved in a crime, and they have every right to question you. You can decline to speak to them if you choose.
If they choose, they can probably get IP numbers for the computer used to purchase subscriptions to the pornographic sites, and perhaps even the IP numbers associated with subsequent use of the purchased accounts. If the numbers don't match those of your ISP, it will be hard for them to make the case that you purchased the accounts.

