You can have opposite sex work friends, but you need limits to keep the friendship appropriate.
Sharing issues about your marriage/relationship to your work husband/wife can just lead to trouble. You are better served by sharing your feelings with your REAL husband/wife and communicating with each other. I’m not a total idiot. Sometimes this ‘sharing’ just leads to more fighting. Well, that’s what girlfriends/the guys are for. When you spend 40+ hours with this co-worker, which is probably more time than you spend with anyone else outside of your husband and kids, you need to have boundaries to keep all your relationships healthy. If you are confiding your fears and dreams to your work spouse that you wouldn’t with you spouse, than you are having an emotional affair. Which can lead to a slippery slope of a physical affair.
Unfortunately, there are skanks and dirtbags out there that solely look out for the ‘taken’ man/woman. There are famous home-wreckers. Angelina Jolie is the first one that comes to mind. I agree that Jolie and Pitt seem happier than when Brad was married to Jennifer Anistan. But, if you are married, then you made a commitment to each other to work through all your issues. If you are attracted to another person other than your spouse, than you should stay away from them! Do not go for lunch and think it will not go anywhere. When lust is in the air, the combination of pheromones, hormones, and mutual attraction is dangerous. Mix in opportunity and forgetaboutit.
I’ve heard some people say “what?! can’t I have friends?!?”. Sure, but if you are spending time with this ‘friend’ that could easily be seen as a date, i.e. you went out for food and drinks, what do you think your answer to this ‘friendship’ should be? Your partner doesn’t feel comfortable with your friendship, than you need to identify what the problem is. Are they jealous? Are they justified in being jealous? Is your friendship with this person worth the arguments, mistrust, and destruction of your relationship?
When you’d rather hang out every other day after work than go home to your family, what are you hiding away from? Get your butt home to your wife and kids!