In fiction, any form of polyamorousl sex is dangerous to a committed relationship.
It is common that if a fictional couple begins dating other people, swinging, or otherwise opening their sexual doors to others, it will go horribly wrong. Much fiction thrives upon the message of the One True Love: the one person that you are destined to be with, and will not only love you for you, but express how much they love you by opening themselves up to you and satisfying you sexually. Under the assumption that people are In Love with Love, the concept of finding "true" love becomes one of the most powerful feelings fiction can produce. A side-effect is the Good People Have Good Sex trope, because if you love someone, then that means you can always fulfill their emotional and sexual needs. Partially to blame for this are Moral Guardians--particularly the Hays Code enforced by Hollywood in The Thirties. The Code would not allow positively-portrayed sexual activities outside of a Happily Married couple. Because of this, most acts of polyamory or extramarital intimacy were either negative or extremely subtle. And so, in many portrayals of extramarital intimacy one (or both) spouses will become addicted to it and start cheating. Or, the third party will become a Stalker with a Crush, determined to win the affection of their already-wedded target. Or, something will simply happen that ensures one member of the relationship enjoyed the encounter, and the other very much did not, with the dissatisfied partner questioning whether it was really worth it. These are just a handful of ways that it can lead to disaster, but the result is the same: it's unlikely that this will be a one-time thing and that the couple just goes on with their lives afterward, with this as just one of many fond memories in a happy marriage. Exceptions notwithstanding, extramarital sex/romance, even when consented by both spouses, tends cause more problems than was Worth It. One of the typical exceptions occurs when the Protagonist is male and the Three-Way Sex is with two women; chances of that having a happy conclusion are plausible. The odds of backfiring double for husbands, fiances or boyfriends who allow their women to stray to other men. At worst, this has the Unfortunate Implications of both insisting that women are still "property" of their husbands, and that any man who thus can't control her, or fails to satisfy her sexually, is not a "real man" at all. The other typical exceptions are "that one couple", who may be swingers, but are portrayed as quirky, odd, or comedic. When that happens, their relationship is never the focus of the story, and they usually only act as Foils to demonstrate how well-adjusted and "normal" the deeply-committed and monogamous main characters are. Enforcing this trope tends to invoke a few others: