I think porn and bitcoin is great than they don't know your bank acc.
The question is, what would they have done even if they knew your bank account?? You would still be over 18 if you had a bank account and things would as usually be legal for you(doesn't matter if they know your bank account or not).
Knowing your bank account is not a matter. But in your bank statement the narration of where you have spent your money would be a privacy problem like when you are planning some loans, you might need to submit your account statements.
Yes that must be a real problem if you use our debit cards or ATM cards in a porn site. Then the statements are no more exposed to public. This might be the biggest reason for many people to use bitcoin for porn sites.