I have a position that is somewhat unusual: I think there are some cases where it is morally acceptable to cheat.
First, the couple has to be really, really sexually incompatible. I am not talking “I wish we could have sex three times a week, but we only have sex two times a week” here, and I am not talking “we rarely have sex but we’re both okay with it” (that is called a mutually satisfactory sexual relationship). Fetishes can be a cause of severe sexual incompatibility: she’s a diaper fetishist, but her husband is absolutely repelled by the concept of involving diapers in sex. Another cause is a very large gap in libido: zie wants sex once a year… maybe… while zir boyfriend wants sex once a week or even more.
Of course, not all relationships with a libido gap or incompatible fetishes are unhappy. If she’s perfectly happy fantasizing about diapers during her sex with her husband, then there’s no issue; if he’s fine masturbating (perhaps while zie snuggles him), then there’s no issue.
But I think a lot of people are unsympathetic to the pain that very sexually incompatible relationships can cause. For a lot of people, sexuality is a core way of expressing affection. They can feel hideous, repellent, utterly unattractive; if the issue is a fetish, they can feel ashamed of their sexuality. They may become depressed. Often, they can grow bitter about their partner; they can feel like their partner doesn’t love them or care about their needs.
Second, there has to be no possibility of a satisfactory resolution. If the lower-libido partner is working on feeling more comfortable in zir body so that zie feels sexier, the high-libido partner is going on an SSRI in a few weeks, the diaper fetishist is expanding her ideas of what she finds sexy, or the diaper-averse husband is trying to learn to appreciate his wife’s arousal, then it would be unreasonable to cheat. Similarly, if the couple is opening their relationship, then it would be unreasonable to cheat. As long as both of you are working on the problem in good faith, you shouldn’t cheat.
Of course, even when people are trying their best, not all diaper fetishists can be turned on by something that isn’t diapers, and not all lower-libido partners are going to want sex more than once a year. And many people are genuinely distressed by the possibility of their partner having sex with someone else. Unfortunately, we do not live in a world in which people can cause changes in their sexuality by really really wanting them.
And equally unfortunately, not everyone understands the importance of sexuality to their partners. Even an otherwise kind and reasonable husband can feel that the problem is their wife having that disgusting fetish and she just needs to learn to like normal sex; even an otherwise kind and reasonable partner can think that zir boyfriend just needs to calm his raging, overactive libido and then everything would be okay. (It can happen that someone claims to be working on the problem, but drag drag draaaaags their feet on actually doing anything; I think the person considering cheating must be discerning about whether their partner is honestly doing their best or just trying to keep them off their back.)
Third, the relationship has to have solid fundamentals. A lot of sexually incompatible relationships don’t, or if they did once it was lost in the conflict over the sexual incompatibility. If you and your partner hurl cruel insults, feel rage and contempt about each other, or are completely emotionally disconnected from each other, then the wisest course is to break up.
For some unlucky people (I’m in this category), having sex with people other than your primary partner doesn’t scratch the itch– it’s not that you want to have sex in general, it’s that you want to express your love and caring to your primary partner through sex with them. I think if you have those preferences a breakup is probably wise.
Fourth, there has to be some reason why it would be very bad for the couple to break up. The classic example, of course, is children. Research suggests that children experience negative consequences from divorce if their parents were in a low-conflict relationship. (Of course, if you and your partner are in a high-conflict relationship, divorce is best both for you and for your children.) For that reason, many people don’t want to divorce while their children are under the age of eighteen. Of course, some people are capable of being celibate for years or decades while they wait for their children to grow up. But some are not. Indeed, for some people, the resentment they feel about their sex life poisons the rest of their otherwise good relationship.
Another example is someone whose partner is dying. Oftentimes, well spouses who have affairs are talked about as uncaring sociopaths who care more about their boners than their spouses. But caregiving is really hard, and illness can make it impossible for the well spouse to have the affection they’re used to. For some people, the stress and grief of their spouse’s illness, combined with their touch starvation, make them short-tempered and nasty to their partners. Having an affair with someone else can be a brief break from the misery that allows them to have their need for touch met; it can make people happier, calmer, and better able to make loving memories with their spouse in the time they have left. Of course, not all well spouses have any desire to have an affair– but some do, and in my opinion, one would have to have a heart of stone to condemn cheating in this circumstance.
Fifth, you must take steps to minimize the damage to your primary partnership. Of course, taking appropriate precautions against STIs and pregnancy is a must, and you must be discreet about your sexual relationships. Equally important is minimizing the threat to your primary relationship– after all, the whole purpose of this is to maintain that relationship.
Consider hiring a sex worker. Hiring a sex worker essentially eliminates the risk that you’ll be tempted to leave your primary relationship for a shiny new lover. If you’re a submissive, prodomming without sexual intercourse is legal in most locations; you can have fetishy sex without risk of STIs, pregnancy, or legal repercussions. (Unfortunately, prosubs are far less common.)
Consider Internet sex. For many people, their desires may be met through flirting, exchanging naked pictures, cybersex, or some combination. That eliminates the risk of STIs and pregnancy and may reduce the risk to your primary relationship, particularly if you establish a persona that’s pretty different from you. Do be careful not to get caught, however.