“Should married Christians have close friends of the opposite sex?”
Everything that can possibly go wrong between a man and a woman starts in the heart. As the Lord Jesus put it, “Out of the heart come evil thoughts, adultery, sexual immorality ...” The heart is where all the bad stuff starts, even if that bad stuff never gets acted out or even discussed in the real world.
An infected heart is a potentially devastating problem for the married Christian couple that has to deal with the fallout from it.