From the Couple's Devotional Bible:
No one seems invulnerable to sexual temptation, and an affair, however "meaningless", has the potential to ruin even the best marriage. Blumstein and Schwartz, in their study of American couples, found that "husbands and wives who had had extramarital sex were more likely to break up, whether the unfaithfulness happened at the beginning of the marriage or after many years."
Comparing couples who had affairs with those who didn't, couples who had committed adultery were less committed to the future together.
A partner's infidelity is more than just a disappointing choice to prefer another person (for a night, or longer). It is a stab in the gut. Adultery threatens any relationship. It betrays the partner who has entrusted life, body and soul to another. It makes him or her into a sexual competitor.
Marriage demands, positively, that we make a pledge to love for life. Negatively, it demands that we abhor adultery. To brush past that rule, emphasizing the positive pledge without the negative command, is to kid ourselves.
Adultery is a marriage killer. That is why the bible is so plain in its comment: "You shall not commit adultery."
But a man who commits adultery lacks judgment;
whoever does so destroys himself.