If a friend tells you that their partner is cheating, your first reaction might be anger. You may have suggestions for your friend that involve swear words, lawyers, and (jokingly, I hope) places to hide the body. We're hardwired to feel outrage whenever someone abuses the trust placed in them by a partner. Outside of very specific kinks, nobody wants to be cheated on. Being lied to, even lies of omission, are hurtful. The idea of our partner enjoying sex or romance with someone else may seem impossible to cope with. But what is it about adultery that makes us so angry? And what exactly does adultery entail?

Is Extramarital Sex Always an Affair?

Earlier this year, a Japanese woman in the midst of divorce sued her husband's mistress, demanding 4 million yen (about $32,000 American). This mistress was the owner of a popular entertainment club in Tokyo, and had sex with the married man regularly for seven years. While the mistress was not paid for sex, she claims she engaged in sex with the man to ensure that he would "continue patronizing her hostess club." The judge concluded that such sex was a "business transaction" rather than an affair. The judge went on to say that the sex was "unlikely to disturb the tranquility of a marriage."

Really? This seems unlikely, since it was the woman's divorce lawyer who initiated the lawsuit. After losing the case, the ex-wife decided not to appeal and said she'd rather forget what her ex-husband had done.

Most of us would refer to seven years of secret extramarital sex with the same woman to be an affair. The club owner claims to have no romantic or emotional attachment to the man in question. So what? There's plenty of precedent for suing someone who knowingly sleeps with a person who is already married.

In Zimbabwe, it is common for cheated-on wives to seek damages against their husbands' mistresses. This often leads to judgments of about $2,000 American. In 2010, a North Carolina woman was awarded $9 million in damages from her husband's mistress.

It seems unlikely that someone could have years of sex with a married man and never consider that it might impact their marriage. At the same time, a mistress has no real responsibility to the man's wife. She didn't make any vows. It was clearly the husband who violated the marital bond.

Recently, affair-enabling website Ashley Madison was hacked - possibly by someone hurt by an extramarital affair. Hackers demanded that the site be shut down or they would reveal customer information, including user connections and the fantasies they posted. Yikes!

Here's the catch, though: Women who sign up on Ashley Madison don't have to give up any traceable information like credit cards. Men do. So, this hack promises to damage cheating husbands far more than cheating wives. That seems more than a little unfair. It's also worth mentioning that Ashley Madison charges around $20 to completely delete accounts and account activity. Some have even suggested that this "hack" is a ruse to net Ashley Madison even more customer monies as men scramble to hide their attempts at infidelity.

Defining Infidelity

When asked what constitutes an affair, infidelity, or "cheating," answers can greatly vary. Some say sex. Others say that platonic romance outside the marriage is just as bad. The word "dishonesty" comes up a lot, whether that means outright lies or mere lies of omission. Cheating is not just for monogamous couples. The poly community is notoriously intolerant of infidelity (which they tend to define as activity that falls outside the agreed upon parameters of a marriage). How individual couples define "infidelity" runs the gamut. Some couples are fine with extramarital sex so long as it's discussed beforehand and is responsibly done. Other couples consider looking at pornography to be an act of infidelity, or that a platonic lunch can be cheating if a partner feels the need to lie about it. (Some would argue that monogamy is an unrealistic ideal. Read more in Is Monogamy Ridiculous?)

No matter who you ask, the core of "cheating" involves some level of dishonesty. Some would argue that there are "big lies" such as what someone did while their spouse was out of town, and "little white lies" like what someone was looking at on the computer after their partner went to sleep. One might suggest that any act of dishonesty can be potentially damaging to a relationship. However, does anyone really reveal every thought, activity, or impulse to their current partner? I have to think that they don't. Telling a partner every minute detail of what we think and feel isn't romantic - or even feasible.

How we define cheating is actually quite personal, and is influenced by our sexual and relationship history, our fears and insecurities, and our expectations for our partners and our current relationships. While researching this issue, I came upon a phrase from a book called "Kosher Sex." With regard to fidelity, it says that "whatever brings a couple closer together is holy; whatever pulls them apart is profane." Some may say that's short on specifics, but I think it's quite elegant. It makes the point that it's up to couples to decide what they value, what they need, and how to communicate that openly and honestly with their partner.