" And so the twins had remained virgins. Julia and Valentina watched all of their high school and college friends disappear one by one into the adult world of sex, until they were the only people they knew who lingered in the world of the uninitiated. "What was it like?" they asked each friend. The answers were vague. Sex was a private joke: you had to be there. "

― Audrey Niffenegger , Her Fearful Symmetry