There's obviously nothing wrong with having friends from the opposite sex. The question
is, however, "How close is the friendship that the two of you would have to make a mutual agreement to remain faithful to your spouses?" When the relationship between two married people who are not married to each other is this close, then temptation, and the possible death of a marriage, is just around the corner.1
It's not wise for one to walk down a path they know leads to death.
1 (James 1:14-15)