Thanks to Kate for recommending Bonk: The Curious Coupling of Science and Sex by Mary Roach. A hysterical romp through scientific articles on sex, covering the last 150 years. Read the book, then go rent Kinsey, if you haven't seen it yet. That Kinsey, he was kooky!
But for the best book on gender, the front-runner remains Woman: An Intimate Geography by Natalie Angier. Fascinating from start to finish. We wimmenfolk, we are COMPLICATED.