Too often we think that relationships with the opposite sex can be harmless. But then you end up going for JUST coffee, and then it's JUST lunch, which turns into JUST dinner, and then it's JUST talking in your car and before you know it, you both find yourself JUST in bed together wondering how this could have happened.
We recommend you Affair Proof your Marriage with Boundaries when it comes to the opposite sex. It's not like you can't have a relationship with them, it just might look a bit different. And the main thing we advise is to NEVER be alone with them. Too many things can happen and in the end, it's not worth throwing your marriage away for the sake of a friendship.