I believe that whether a married person should or should not have a close friend of the opposite sex comes down to the definitions of "close and "friend."
Close friends play an essential role in our lives. They often share our dreams, hopes, humor, and beliefs, leading to greater affection and deeper feelings of connectedness. This warm affection brings us comfort and joy along the journey of life. I believe that having friends, even of the opposite sex, is normal and healthy. However, where there is some debate, and clearly an increased risk, is in regards to having a close or intimate friend of the opposite sex.
I believe that close friendships for a married person with someone of the opposite sex are riskier because such affection, connectedness, and familiarity usually deepens our emotional bonds, and can lead to increased levels of romantic feelings. It is in this place where the risks to one's marriage are most acute. When a person marries they make a commitment to one person, a vow of friendship and faithfulness. Such a vow encompasses many areas, but especially emotional and physical intimacy. This does not mean that outside friendships must cease, but it does mean that any intimacy shared must have well-defined boundaries. And there must not be any secrets from our spouse, as this can undermine trust, the very foundation of our marriages.
And so in answer, friendships are a good thing, even with members of the opposite sex. But as friendships become more intimate, romantic feelings can spring up. And thus, great care must be taken when a married person has a close friendship with someone of the opposite sex. If you are interested in more information, I have written a blog on this topic, which is linked here: The Risk of Opposite Sex Friendships in Marriage.
Christopher Grace serves as the director of the Biola University Center for Marriage and Relationships and teaches psychology at Rosemead School of Psychology. He and his wife, Alisa, speak regularly to married couples, churches, singles and college students on the topic of relationships, dating and marriage. Grace earned his M.S. and Ph.D. in experimental social psychology from Colorado State University.