Another word for infidelity is unfaithfulness and it means that a married person has been unfaithful to their spouse by having a relationship with someone else. Infidelity can happen both sexually and emotionally – in other words it is possible to be emotionally unfaithful to your spouse without actually having intercourse with someone else. This is definitely a very common reason for marriage failure; if not sexual infidelity, certainly emotional infidelity as someone or something else becomes more important than the marriage relationship. Instead of loving and cherishing each other as husband and wife, they allow other things or people to crowd out the love in their marriage.
Marital fidelity is usually concerned with monogamy (sexual loyalty), and the breaking of those vows would be considered infidelity in the eyes of the law. Decades ago, when state laws required divorcing couples to cite reasons and find liable parties as part of the divorce proceedings, data on the most common reasons for divorce were easier to come by. However, since most states no longer require petitioners to cite reasons for wanting a divorce as long as it’s agreed by both parties, information about why couples split has become more fragmented and harder to find.