I dont see any negative impact towards the future of Bitcoin if majority of the porn site accepts Bitcoin as an option for payment. In fact, it will help Bitcoin to spread it's popularity and adaption, because people will understand the value of cryptocurrency in the field of currencies.
Debit cards, CC, and other form of digital currency was use to purchase or payment for premium/VIP accounts in all pornsites and is actually nothing wrong with that. Same case with Bitcoin.