Should a married person have a close friend of the opposite sex?

TL;DR:

Opposite sex relationships can be beneficial within proper boundaries. Married people with friends of the opposite sex should reflect love, purity, and respect, while investing in and prioritizing their marriage.

what does the bible say?

There is no place in the Bible that forbids men and women from being friends with each other. Friendships with the opposite sex are acceptable, even beneficial, within proper boundaries. But boundaries are necessary. Many people who have affairs would never have anticipated cheating on their spouses, but it still happened. Why? Because there were not proper boundaries in place. Married spouses should have mutually agreed upon boundaries that will protect their relationship and maintain their mutual trust.

from the old testament

The Old Testament does not address a married person having a close friend of the opposite sex.

from the new testament

Friendships with the opposite sex are acceptable and can be beneficial, when kept within proper boundaries. We are called to love one another—not just women with women and men with men, but everyone. However, the call to love one another also comes with a boundary: we are to do so with purity and respect (John 13:34–35; 1 Timothy 5:1–2).
With marriage comes great joy and blessing but also sacrifice. Things that may have been perfectly acceptable as a single person become unacceptable as a married person. Some have described marriage as one big "yes" that comes with a lot of little "nos" attached. You are no longer your own (1 Corinthians 7:3–4).
Philippians 2:3–4 says, "Do nothing from selfish ambition or conceit, but in humility count others more significant than yourselves. Let each of you look not only to his own interests, but also to the interests of others." This is true of married couples and friendships alike—what is in the best interest of your spouse? What is in the best interest of your friend? These considerations are crucial for your marriage, and they are just as important for the friend in question.
All that we do should be done for the glory of God (1 Corinthians 10:31), so we shouldn't put ourselves in contexts that could head in an unhealthy direction or be misconstrued in their intentions. Whether your close friend is married or single, you also have to consider the space you may be occupying in their life and heart. Are you positioning yourself to create or meet needs in their life that you are unsuited to fulfill?
You can build a marriage on your foundation of faith in Christ, but that still won't make you immune to extra-marital temptation. The Bible instructs us to flee from lust and sexual temptation (2 Timothy 2:22). This instruction is not just for single people; it is for all people.
Marriage doesn't cure lust and temptation. We all need to proactively flee from sexual sin, because that is a better option than trying to stand our ground and fight it (1 Corinthians 6:18). When unfaithfulness of any kind has occurred, it requires great time, consistency, and effort to rebuild the trust that was lost.

implications for today

When times in the marriage are good, it's easy to maintain external opposite sex friendships because you are feeling more fulfilled by your spouse. However, when times get tough in a marriage, it can become a temptation to develop unhealthy emotional attachments, if not physical ones. Friendships require emotional investment, which is enriching and important; however, we have to be wise about with whom we form close emotional bonds. If a woman has a male best friend, not her spouse, she will most likely want to go to him as a confidant, advisor, and source of support when times are tough. The same is true for a man with a close female friend.

When a spouse is distant (physically or emotionally) and we have another friend of the opposite sex, it can be tempting to fill in the gaps of what is missing with our spouse with that person, which can lead to those classic "in love" feelings. As tough as it may be, it is a wiser choice to communicate directly with your spouse when you are having struggles rather than seeking comfort from another. Consider the question, "Is this friend meeting a need for me that my spouse is a more appropriate person for?"

Married men and women should do their best to avoid any potentially compromising situations or relationship dynamics with friends of the opposite sex. They must develop healthy boundaries that work for both the husband and wife. And overall, both the husband and wife must invest in their marriage, communicating, growing, and loving well together.

understand

Friendships with the opposite sex are acceptable but require clear boundaries to protect the marriage.
Married individuals should prioritize their marriage, potentially reevaluating friendships to avoid emotional or physical temptations.
The Bible advises fleeing from temptation and sexual sin, highlighting the need to avoid compromising situations that could threaten marital trust.

reflect

How do you currently define the boundaries in your friendships with the opposite sex, and what adjustments might you need to make to prioritize your marriage?
Have you ever found yourself emotionally leaning on a friend of the opposite sex during tough times in your marriage? How can you shift that reliance back to your spouse?
How can you actively communicate with your spouse about your friendships to ensure mutual trust and understanding?

engage

What are the potential benefits as well as pitfalls of having friendships with the opposite sex?
What biblical principles can you apply to ensure that your friendships with the opposite sex remain pure and respectful?
Reflecting on 1 Corinthians 10:31, how can you ensure that all your relationships—especially those with the opposite sex—glorify God?