The basic fact about cheating is the lack of interest in each other. A person cheats in a relation when he or she finds someone else to be better than his or her partner, in certain ways. And when this case arises, besides letting someone else substitute his or partner, the person also deeply hurts his or her partner.
Cheating in a marriage goes against the idea of marriage. Two people happily committed to one another will not look outside the marriage. If they do, then the marriage should end. One party is clearly unhappy enough to be unfaithful, therefore the injured party should not remain married to someone who has lost feelings toward them.
When vows are taken, they are meant to be upheld, in good times and in bad. When one spouse cheats, they have made a mistake. We all make mistakes and sometimes we regret those mistakes. A divorce should not happen when one regrets their mistake. However, if the cheating is habitual and the cheating spouse is just playing around and is not taking their marriage vows seriously, then a divorce should be an option.