Just try to include your wife in your activities so you dont have to worry about not being fair to her. You definetly should watch out for someone to start growing feelings for the other. It's inevitable that feelings will develop. But if your wife really doesn't mind then don't sweat it.

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One day the Moon said to me, "If he makes you cry, why dont you leave him?" I looked up at the Moon and said, "Moon would you ever leave your sky?"