Here's my answer to this question: Sitara and others have already answered your question. Let me add something that I FEEL is missing in their answers: Erotic carvings are nothing but codes- Tantric codes/mantras. Each act represents a sound such as "Ahm" "Hram" "Krim" and others. So to be a real Tantrik, you have to be able to decode these hidden codes. If you are really into unlocking the mystery, Rathi Dhan Shumsher's "Kama Kala Rahasya" published by the Royal Nepal Academy sometimes around 2028-2032 BS, might be a good place to start. Happy decoding!