I know that I sincerely wish my ex-husband had had close friends that he spoke to often, of any gender. His isolation was a major factor in our divorce and I think still a very tragic strain on his mental health. For the record, infidelity or outside friendships were not, to the best of my knowledge, a factor in our marriage ending. On the contrary, I think friendships like that were a big part of why we lasted fifteen years together.
I find the idea that men and women in committed relationships shouldn't ever grow close frankly terrifying. Being the sole confidant and emotional support to each other is too much weight for a relationship to carry. Who do you turn to when you are both under a strain together? Who shares your shared joys?
I think from time to time, in any committed relationship, some sexual, romantic, or emotional attraction to someone else will arise, and if you can non-judgementally inquire into it, it will pass. For me, over the course of my fifteen-year marriage, sometimes it was about something that I wanted to be or express that wasn't currently part of my life. Very rarely it was about something that wasn't a part of my relationship, that I tried to address without bringing the other person into it, i.e., you never hold the car door for me like my buddy Alan does.
But sometimes it was just that one of us had met a cool person that one or the other of us wanted to be friends with. I can think of one woman my ex was warm friends with with and I while I did occasionally feel some jealousy, I also remember thinking how great it was that he had this friendship, and seeing how edifying it was for him to have it. If you can talk about jealousy or ask for reassurance without getting into a lot of how-dare-you-not-trust-me, that's a good thing to do.
A key ingredient was that I felt confident in my ability to exercise sexual self-control, as well in as my now-ex-husband's. Neither of us were the type of person who referred to hookups ''just happening'' or felt that feelings we might feel meant we were bound to act on them. Risks exist, but like in everything else in life, risks are sometimes necessary and worthwhile.
It's also important to keep a bead on the outside friend's feelings, for their sake and yours.