Sex was primarily a survival and defence mechanism. A pleasure that is pain, romanticised. Copulation was a tool to reproduce without which a species would perish. Somehow we got it under our control; we tracked and understood cycles.
Sex was a necessity. Today, it has turned into a need. Human mind and consciousness attached emotions to it, that’s how agony turned into gratification.
We choose to deny but everybody seeks pleasure in pain. How they do differ in ways.