That is not to say that adultery is justified, or that it is acceptable to stray when one feels more love from a paramour than from a spouse. It is to say that a marriage in crisis because a spouse feels loved by another usually involves a serious problem in the marriage. Whatever the symptoms, the underlying cause probably relates to at least one spouse, if not both, feeling a lack of love because they are afraid to let the other person see into them. Or they did let the other person see into them but were rejected, castigated, or made to pretend to be someone else.
Intimacy is the answer to a great many problems in relationships. However, it will never occur until a person feels safe enough to show who he or she is, and the other person in the relationship can accept them as they are.
Without that, all relationships are temporary. They exist until a person finds acceptance and love from someone else. Don't let that happen to your marriage.