You could try The Origins of Sex, in which Faramerz Dabhoiwala argues sex was invented in the eighteenth century.
The Origins of Sex shows that the creation of this modern culture of sex was a central part of the Enlightenment, intertwined with the era’s major social, political and intellectual trends. It helped create a new model of Western civilization, whose principles of privacy, equality, and freedom of the individual remain distinctive to this day.Or the Guardian Online's survey of Valentine's Day wishes from the past (example above).
Or if you are really not big on the hearts-and-roses stuff, Mimi Alford's Once Upon A Secret continues the unveiling of President Kennedy as a sexual predator.
And if sex is only 400 years old, apparently heterosexuality is even newer, according to Hanne Blank's Straight: The Surprisingly Short History of Heterosexuality, noticed here.
