Infidelity is one of the most harmful problems that can come up in a marriage and often leads to divorce. It usually comes about as a result of several components being missing in the partnership... but often it's not due to a lack of sex. It's more about the lack of intimacy between the couple. Whatever the reason, one or both partners decide to stray and seek comfort from an outside person.
There is no question that turning away from your partner to another person outside of the relationship, is no way to go about fixing the problems and trying to save the marriage. As someone involved in a relationship, you owe it to yourself to understand the possible signs your partner is cheating on you.