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

The Bible does not forbid close friendships between men and women. As
Christians, however, there are some principles that we would be wise
to heed. Married people especially need to be wary of friendships with
members of the opposite sex because temptations are more likely to
arise when there are marital problems. If a man's best friend is a
woman who is not his wife, he is likely to share these problems with
her, which can lead to an unhealthy emotional attachment. The same
holds true for a woman who has as a best friend a man who is not her
husband.

Most married men (or women) who have affairs don't purposely go out to
find a romantic interest outside of their marriage. Many people say,
"I didn't mean for it to happen; it just happened." But these things
"just happen" when we "play with fire" and put ourselves in situations
that are difficult to control. When we feel a spouse is not attentive
to our needs, we can easily feel that we have "fallen in love" with
someone else who does give us the attention we crave. When we feel
ignored or under-appreciated by a spouse, we should communicate our
struggles with the spouse and avoid the danger of seeking comfort
elsewhere.

Even a marriage that is built on a foundation of faith in Christ and
has relatively few problems is not immune to extra-marital
temptations. This is why the Bible does not tell us to stick around
and try to fight temptation, but to flee from it like we do from all
"youthful lusts" (2 Timothy 2:22). Trying to fight temptation seems to
become especially difficult when it comes to matters of the heart or
the lusts of the flesh. First Corinthians 6:18 tells us that we need
to run away from sexual sin, because it is much easier to run away
from temptation than to stay and fight it.

Married men and women should carefully avoid putting themselves in
compromising situations when it comes to the opposite sex. If they are
seen together in public, it will give the wrong impression. If they
are alone on the phone or in person, they will subject themselves to
the temptation of an emotional or physical affair. The Bible tells us
that everything we do should be for the glory of God (1 Corinthians
10:31), so the wise thing would be to stick to visiting as couples or
"double dating" with other married couples, as opposed to risking the
complications associated with close friendships with the opposite sex.