I think it represents a more taboo set of thoughts / actions than it's milder alternatives. Fill in the blank: Illicit lovers ____ where married people have ____ and young lovers make ____.
PS: It's not accurate but many people still think the world started with: "Found/ Under Carnal Knowledge" or "For Unlawful Carnal Knowledge" which comes back to the idea that the word means more than just the act of procreation.
Interestingly, I find that some of the more creative "euphemisms" for sex (pork is a middling example) have more shock value when they show up in conversation... the old standards are simply overused, in spite of our best efforts in child rearing, and don't work anymore in conjuring up sufficiently taboo images. Terse, vivid imagery is much more of a live wire.
PS: It's not accurate but many people still think the world started with: "Found/ Under Carnal Knowledge" or "For Unlawful Carnal Knowledge" which comes back to the idea that the word means more than just the act of procreation.