Note: This post discusses explicit sex in an explicitly sexual work, and includes images which are not safe for work.
It ought be fairly clear that most pornography is not an accurate reflection of reality. But perhaps more interestingly, different pornographic works inhabit realities which are vastly different from each other. When they divorce themselves from real-world sexual mores, erotic texts are often freed to reinvent sexual morality from scratch.
This is true to all fictional works up to a point, but the dissonance between how strongly sexuality is highlighted in pornography and how often guarded or shameful it is in reality makes the departure consistently, explicitly more noticeable. That said, so long as the author inhabits the real world and real sexual mores, their departure from those mores is clearly marked as a departure.