When it comes to having a friend of the opposite sex and not involving or including your spouse in that friendship that is where problems arise.
Speaking from experience, when I found my spouse to have a friend of the opposite sex; and he did not want to include me in this relationship, it turned out to be that a relationship with this woman. He was sharing things with her, that he should have be sharing with me. The more time he spent with her the more distant he became with me. The friendship became a 10 year affair. It hurts to the very depths to find that you are not your spouses friend.
Guess what I am saying; if you have a friend of the opposite sex, share that person with your spouse and be straight with the relationship. I enjoy friends of the opposite sex, but we cheat ourselves when it is not both your friends. This is just my honest opinion at 3 o'clock in the morning and I should be sleeping.