If you want to understand what women want, don't ask them about their relationships; ask them about their affairs.

In writing my book The State of Affairs, I came to realize again and again that illicit relationships offer a window like no other into the mysteries of female desire. Perhaps this is because, in the context of marriage and committed relationships, women are still accustomed to doing things according to cultural norms and expectations — whether due to pressure, obligation, or simply as part of a trade-off.

What women do in marriage tells us less about what they want than about what they value. In their affairs, however, we get a penetrating glimpse into their free will. Far be it from me to justify infidelity, but as a seeker of truth, I have come to find the truth often hides in places that are less comfortable.

A woman I'll call Madison, 31, has been living with a man I'll call Steve for almost five years. They met at the Brooklyn coworking space where she runs her startup. She tells me that she loves Steve and still believes they'll get married and have a family in a few years' time. But two months ago, she reconnected with her college boyfriend on Facebook, and they've been hooking up.

When I inquire as to why, she tells me that in the last year, she has lost interest in sex. "I used to be so into Steve," she says. "And I was often the one who'd jump on him when he came through the door. But then it just became such an effort, I couldn't muster it up. It really freaked me out. I'd do it because he wanted to, and sometimes I'd get into it, but mostly I felt numb. I reached out to my old flame to find out if I had really lost that part of myself."

Madison discovered that her playful, erotic self was far from dead. In my conversations with her, we explore the fact that she often finds it difficult to hold on to her own identity in the context of her relationships. In her affair, however, she knows for a fact that she is doing what she actually wants. She's not taking care of anybody; this is just for her. Secrecy becomes her pathway to autonomy. She is no longer playing a culturally sanctioned role — the nice girl, the girlfriend, the wife, the mother. Through talking to women like Madison about their affairs, I've observed a few themes about their sexuality:

1. Women tire of monogamy faster than men.

One of the most widely held beliefs about women's sexuality is that it is rooted in security and commitment.

It's commonly thought that men are not really wired for monogamy while women are more naturally inclined to be content with commitment or exclusiveness. Men, the theory goes, need novelty and variety in order to feel turned on, while women need closeness and attachment. Researcher Marta Meana invites us to question this assumption.

If it were true that women's sexuality is primarily dependent on relational connectedness — love, commitment, and security — then shouldn't sex be thriving in loving, committed relationships? But too often, it's not. Take Madison, for example, and the countless other women like her who've reached out to me in recent years. In many cases, though surely not all, when the spark dies, it's a woman who shuts down first and loses interest in her partner — male or female.

Meana suggests that in fact, "Women may be just as turned on as men by the novel, the illicit, the raw, the anonymous, but the arousal value of these may not be important enough to women to trade in things they value more (i.e., emotional connectedness)." As I have often said, our emotional needs and our erotic needs do not always neatly align. But women are well trained to put their emotional needs ahead of their erotic needs — they have much to gain from choosing stable relationships over sexual pleasure. It doesn't surprise me that Madison still intends to marry Steve — but it also doesn't surprise me that she's reconnected with the ex who wasn't "husband material."