The answer seems simple, not worthy of 306 pages, hardcover binding and prime real estate on bookstore shelves.
But the reasons women have sex -- from pleasure and intimacy to revenge and competition to self-loathing and sheer boredom and beyond -- are as complex as the female sex itself, as the new book "Why Women Have Sex: The Psychology of Sex in Women's Own Voices" reveals in juicy detail.
The book reads like a clinical version of "Sex and the City." Drawing from five years of research and an online survey of 1,000 women, University of Texas psychologists Cindy Meston and David Buss splice anonymous first-person anecdotes of women discussing sex with academic musings on how evolution, society and biology all play into female sexual desire.
The result resembles a cocktail-fueled conversation with your best girlfriends and Dr. Ruth Westheimer -- you come away learning things about women you never expected, and maybe understanding a bit more about yourself too.
"We found lots of surprising things," said Meston, director of the University of Texas at Austin's Sexual Psychophysiology Lab. "The overall biggest surprise is the huge diversity of reasons why women said they have sex. We certainly expected more than love, reproduction and because it feels good, but all the different ways women were having sex was incredible."
"A lot of women had sex simply for the adventure, the excitement," she said. "They wanted to try out men of different ethnicities, different penis sizes. And some did not want the emotional attachment. We did not find strong support for that theory at all. Physical gratification was placed much higher."
Some of Meston and Buss' theories are surprising as well. One: "mate-poaching" -- the notion that single women naturally want to steal smart, attractive, personable men away from their girlfriends and wives, and sometimes go to incredible lengths to do so. (Of course, as the book notes, men poach mates too.) By advancing the theory and legitimizing the act, are the authors condoning cheating and coveting thy neighbor?
"If by rationalizing you mean explaining, then yes, but justifying, no," Meston said. "It's still a bad thing to do. But the bottom line is men and women mates poach because a lot of times the good ones are taken up. It is the case that if a good-looking guy has a good looking partner, then he must be doing something right to get this good-looking partner. She's already preselected him, and this in some ways makes him more desirable."
Another unexpected conclusion from their research:
"A lot of women report that the moment of penetration is the most pleasurable aspect of sex," said Buss, who previously published "The Evolution of Desire" about varying cultural attitudes about sex. "Orgasm is certainly up there, it's in the top three, but I don't know if it's the No. 1 physical reason."
But the authors' best findings come in the form of the anecdotes gathered from their vast online survey. Identified only by their ages and self-determined sexual orientation, the women behind the tales give the book meat and set it apart from myriad other sexplanation tomes on the market.
Below, 13 of the most revealing, raw, at times gut-wrenching reasons why women really have sex:
To Get Closer to God
"It was a dream come true, being with this incredible man. I was able to lose myself and see God, where the edges of the dreamworld and the real world met." Heterosexual woman, age 23