When most people commit to a relationship, in marriage or otherwise, fidelity is one of the basic tenets of that commitment. A relationship in which people are free to date or even sleep with others is usually qualified as being "open" or as having "an understanding." As such, sleeping with or dating others is a clear affront to the fidelity of a relationship without the consent of the partners involved. However, does the same logic apply to the sexual activity in your mind?

In the book, Who's Been Sleeping in Your Head? The Secret World of Sexual Fantasies by Brett Kahr, the author discovers through interviews and research that approximately 90 percent of all marriages have been subject to "intra-marital affairs," or the concept of fantasizing about someone other than the other person's partner. Is it fair for him to equate fantasizing to a type of affair, though? A review of the characteristics of these fantasies and the reasons for having them might help to shed some light on the subject.

Common Themes in Fantasies

As one may expect, there are as many types of fantasies as there are people. However, Kahr found that there are some common themes that fantasies exhibit, including:

• romance
• violence
• illegal activity or unlikely
• homosexual
• group sex
• voyeurism and exhibitionism
• object love

While there are some people who claim that they do not have fantasies, Freud would have likely disagreed. In one of his many theories, Freud suggested that people link sex and violence in their subconscious, and are "turned on" by both. As such, fantasies can be a way for people to satisfy their aggressive urges. Since violence carries many negative connotations, it could follow that those who are reticent to admit their fantasies are doing so because they think they are wrong.

Why People Fantasize

Perhaps the most common reason for fantasizing about others, as identified by Kahr and a variety of other sources, is that it helps to make sex more exciting. Many couples who have been together for extensive lengths of time can find sex to be routine or boring. In these situations, sharing or acting out fantasies can be a great way to liven up sex with their partner while staying within the confines of their relationship and their bedroom (sometimes). Couples don't necessarily need to share their fantasies in order to achieve this effect, however. Men and women can use fantasies to manage their personal sexual experience and levels of arousal. Men might alternate between thinking grotesque or benign thoughts and generating hot fantasies in order to increase their longevity during sex, and women can use the same technique to control when or how often they orgasm with a partner. 

The man's use of fantasies to increase performance can also be attributed to a speculation called the "Coolridge Effect," which suggests that males actually exhibit improved levels of sexual performance when presented with a variety of willing females. As such, men who fantasize about other women while having sex with their partner may actually perform better than if they were to simply fantasize about or focus on their partner.

Wish fulfillment is probably the simplest reason why people in relationships will fantasize about others. This can include fantasizing about sleeping with celebrities, sleeping with multiple people, or being in other stimulating circumstances that are difficult or impossible to replicate. Fantasies don't just give people a way to enjoy implausible situations, however. They offer a safe place to try new things or work through sexual issues. It's impossible to catch a disease from a fantasy, and it's a safe environment for a person to gain a sense of experience and comfort with any sexual acts which would otherwise cause them anxiety.

When Fantasies Become Reality

Though fantasies occur in the mind, they are centered around physical acts that could oftentimes be carried out in real life. In the sense that fantasies can help a person get comfortable with new or different sexual experiences, it can also become its own source of boredom. Repetition of the same fantasy can leave a person wanting more, which can, in turn, cause them to take it to the next level.

However, boredom alone is typically not a sufficient reason for partners to bring a fantasy into fruition. More often, infidelity takes root in accompanying relationship issues (Kahr), including:

• Exploration—If a person wants to try new things but they find it difficult to communicate this with their partner, or their partner is simply unwilling, infidelity can present itself as a way to resolve the resulting dissatisfaction.
• Escape—If a person in a relationship is depressed or dealing with other psychological effects, or if their partner is overbearing or abusive, a fantasy brought to life can also bring reprieve.
• Retaliation—The infidelity of one partner will likely cause pain in the other, making it an especially biting vessel for delivering retribution.
• Exit—For the same reason that infidelity is an effective way to get back at one's partner, it can also be a way for one person to cause an end to their relationship without directly breaking it off.

Infidelity in a relationship is usually regarded as a justifiable cause for a breakup or divorce. At minimum, both parties will have to struggle with a break in trust and emotional distress. If children are involved, the family dynamic can suffer as well.

Though physical affairs are common in situations of infidelity, sex doesn't have to be involved for one to act out an affair. Emotional affairs can occur when one person shares intimate details or otherwise becomes emotionally involved with a person outside of their relationship. Though sex doesn't always follow these types of affairs, they can be damaging to a relationship nonetheless.

In an emerging number of incidences, fantasies materialize within the confines of virtual reality. Today, people can live out fantasy lives in massively multiplayer online role-playing games, or MMORPG, such as World of Warcraft or Second Life. While acting out such fantasies doesn't necessarily involve interaction with another person, it can monopolize the time and energy of a person enough to cause as much damage as a sexual tryst might.

Fantasies and Society

It is probably not surprising that a number of religions discourage fantasizing about individuals outside of the marital relationship, including Islam and Christianity. Still, Kahr found in his research that most of the reports of "intra-marital affairs" occurred when people were masturbating. Conversely, two-thirds of those surveyed said that they fantasized only about their partner during sex (the 90 percent statistic at the beginning of this article refers to sex and masturbation combined). So, even though Kahr's statistics initially paint a dismal picture of long-term relationships and fidelity, it actually seems to affirm the sanctity of committed sex in the end. And if a little creative fantasizing can make sex more fun now and then, what’s the harm?