"Fifty Shades of Grey" is a hugely popular erotic best seller with a film version recently released. This book, which started out as "Twilight" fan fiction, is the U.K.'s fastest-selling paperback book of all time. Dubbed "mommy porn," the book has surpassed even "Harry Potter" in sales, with over 100 million copies worldwide.

One reason for its popularity is its depiction of sex, in our society, as abundant and highly charged. This myth is indicative of our prevailing culture. As women, we think we should be full of desire, and "swept away" by all sexual encounters. If we aren't, or don't, then there must be something wrong with us.

But the reality of women as sexual beings is quite different: 40 percent of women in the United States report sexual concerns and 12 percent say they have sexual problems that cause them distress.

Sex researchers Masters and Johnson first described their linear model of the human sexual response cycle in 1966, with their groundbreaking book: "Human Sexual Response." They described the "normal" response as consisting of four phases, starting with desire and progressing through arousal, orgasm and then resolution.

This model was a good start, but recent research has revealed that for many women and some men, our phases may vary in sequence, overlap, repeat or be completely absent for all or some of our sexual encounters.

Libido is the word we use to talk about our desire to have sex - otherwise known as our sex drive. If we have a high libido, this means we are thinking about being sexual or feeling "lusty" and have a "good" sexual appetite. But what about folks who have very low libido? If "normal" sexual activity is always portrayed as starting with desire then what?

Having low desire is the most common sexual problem in women, reported by 39 percent, and in my practice, I certainly hear this complaint frequently. Many women experience a decline in their sexual desire at menopause, with increased stress in their lives, as well as other interpersonal factors. Understanding why libido may be low and giving permission to start sexual activity from a sexually neutral space can be liberating and often is helpful in accepting our current state of sexual function.

There are a lot of reasons why someone may want to be sexual, even if they have a low level of desire. Women are often motivated by a wish for emotional closeness and to strengthen a relationship with a partner. A person's satisfaction with their sexual experiences does not require achieving all four phases, and desire may not be present initially, but may increase in response to pleasurable activity.

So my advice is, if you read or see "Fifty Shades of Grey," enjoy the story but don't feel like the Lone Ranger if it doesn't reflect your experiences.