Many people offer theories explaining why adultery happens and what factors make it more likely. Some argue that monogamy is unnatural, therefore cheating should not be surprising. But theories can't tell us whether adultery is right or wrong, because such judgments do not depend on explanations. Scholars can explain why it can be hard to stay monogamous, but we wouldn't even try unless we had a reason to, and most of the time, that reason is moral.
Before we plunge into the morality of adultery, let's agree on what adultery is. If we define it by what counts as cheating, we're stuck, because different things imply cheating for different people. For some, it may be just sex, but for others it may include kissing or even coffee with another person.
Cheating is whatever one's partner is uncomfortable with. Each partner has the right to set boundaries for what is acceptable, and the other person has no right to violate those boundaries. If you don't like the boundaries your partner sets, then either talk about it or leave, but don't stay in the relationship while doing things that you know will upset your partner. No one deserves that.
However it is defined in any relationship, most people—including ethicists—agree that adultery is simply wrong. Adultery involves the breach of a commitment, a broken promise by one person to be faithful to another (according to the boundaries upon which they agree), which is a basic violation of trust. It also usually involves deception more broadly, since the adulterer must sneak around his or her partner's back. And as a result of the betrayal and lies—which are usually considered wrong in and of themselves—adultery simply hurts the other person, adding personal harm to general wrongfulness.
However, simply saying that adultery is wrong is not much help when it comes to specific cases. All the things about infidelity that make it wrong allow for exceptions themselves. Lying is wrong, but we can think of situations in which it may be justified, such as to save a life or to avoid hurt feelings. Promises should not be broken, but there may be cases in which something more important is at stake, such as an emergency. And causing someone pain is always bad, but sometimes cannot be avoided. There may be reasons to compromise a moral principle, but they have to be good reasons. This is the only thing that keeps us from rationalizing every bad thing we do—including cheating.
What kind of reason might justify violating a moral principle like fidelity? It has to be a more important principle, one that outweighs the first in the person's judgment. The principles that tell us not to lie, break promises, or hurt people guide us to consider the feelings or interests of others, and are usually considered more important than simple self-interest. That's why "because I wanted to" is never a good reason to do something wrong; as moral principles go, it's awfully weak!
Nonetheless, there are valid moral principles that are self-focused, the most essential being that of self-preservation. As important as concern for others is, we have a responsibility also to look after ourselves, both physically and emotionally. If a person is in a horrible relationship, which he or she cannot leave for some reason (financial, perhaps, or concerning children), and feels an affair would help to endure the situation or perhaps get out of it, it would be hard to dismiss an affair as immoral out of hand. This is a case in which staying loyal to one's partner comes at a greater moral cost—to oneself.
It is no contradiction to say that adultery is wrong in general, but there may be extraordinary circumstances that justify it. There's a lot of room between "nothing is allowed" and "anything goes," and sound judgment must draw the lines in between. It's great to have firm moral principles, but only if we realize that sometimes the cost of sticking to them is too high—especially when that cost is a more important principle. Ethics can help determine what the principles at stake are, but each person must use his or her judgment to decide which ones are most important. And when it comes to using judgment, there is definitely no cheating allowed!