Oct. 14, 2009 -- 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
To Even the Score
"My husband cheated with my best friend, so I had an affair with her husband for three months. I did not feel guilty at all." Heterosexual woman, age 44
Out of Pity and and Want for Power
"I had sex with a couple of guys because I felt sorry for them. These guys were virgins and I felt bad that they had never had sex before so I had sex with them. I felt like I was doing them a big favor that no one else had ever done. I felt power over them, like they were weaklings under me and I was in control. It boosted my confidence to be the teacher in the situation and made me feel more desirable." Heterosexual woman, age 25
Because They Were 'Out of Her League'
"I have specifically gone out with PhDs, MDs, JDs [and] CEOs because I have always thought they were out of my league. Thinking they are so much smarter than I am because of a degree. I really wanted to know what makes them tick and what makes them better than I am. I have slept with all of them. I learned they are no different than a mechanic or factory worker; they just have more money." Heterosexual woman, age 42
To Squash the Competition, Real or Imagined
"My boyfriend is always telling me how gorgeous he thinks Kate Moss is. Really, she is just a skinny, drug-addicted b***h. What method did I think about using to kill her? I thought about taking a wire coat hanger and putting it through her eye to make her brain dead. Then I would hang her skinny body up in my closet and show my boyfriend that she isn't so gorgeous after all." Heterosexual woman, age 20
"I only [have sex for money] with my kid's father. Not a prostitute or anything and because I love him but nothing is free in this world." Heterosexual woman, age 32
Because Everyone Else Was Doing It
"I was in college and all my friends had experienced [sex] and I wanted to know what it was like. Thinking about how everyone in the world knew what sex was and that people started wars and killed over it ... it made me curious and I felt a sort of 'pressure' to find out about it." Heterosexual woman, age 24
Out of Boredom and Want for a Good Meal
"I was seeing a man who was 69 years old, 22 years older than me. He took me to an expensive steak and seafood restaurnt. I was only seeing him because I was bored, new in town, had not met anyone else. We both lived with relatives so mostly parked in his car, a big Cadillac. He usually wanted oral sex, so I did it. I figured, why not? He enjoyed it and I got a good meal." Heterosexual woman, age 47
Because of a Sense of Duty
"When I was single, I had sex for my own personal pleasure. Now that I am married, I have sex to please my husband. My own pleasure doesn't seem as important as his. I believe he feels the same way." Heterosexual woman, age 26
To Burn Calories
"My boyfriend and I were on a diet and felt we were not getting enough exercise so we decided to have a little fun and burn a few extra calories a few times a day by having sex. It was fun and really helped us to feel the burn."Predominantly heterosexual woman, age 25
In Place of Painkillers
"I suffer from migrane headaches and although the attacks are few and far between and are generally mild, I find that when I have sex during my headaches, especially when I have a great over the top climax, it goes away before I know it." Heterosexual woman, age 42
"Being completely in love with another where you want to become one -- one inside each other -- spiritually and physically, exploding to become inside out." Gay/lesbian woman, age 43
Because of Love
"Um ... is there any other reason to have sex? Seriously. Love is pretty much it, as far as I'm concerned." Heterosexual woman, age 35