I would consider my DH having a relationship like that with a female coworker to be a form of infidelity.
Ummm.... it really isn't. If you have a true work spouse, not a work crush. I have a male co-worker that I have worked with for 9 years. For most of this time he was not married, but I was. He got married about 3 years ago and had his first child about 18 months ago.
We work in the same office (I'm actually his supervisor) and share a court division together, so we are together alot and we depend on one another. I probably know him personally almost as well as his wife and he knows me that way too. There are a few personal things I haven't shared with him and I'm sure he is the same, but most things he knows.
He has shared my girls growing up. He wants to know all about them and genuinely cares for them and my husband. I have helped him with his girlfriends, his wife and miscarriages and his child being born. We are very close, like a best friend but, I can tell you I have never, ever had for one moment, a thought of anything else with him but friendship. He is like my best friend in a way, but nothing has ever even been hinted at in a sexual manner.
My DH knows him and likes him and knows we are close. His wife also knows we are close. Neither one of us, as far a I know, have ever had any issues with this with our spouses.
We do not communicate outside of work though, so maybe that would make a difference. We never go for drinks, talk on the phone or anything like that. We do go to lunch sometimes, but it is always with a group. He will every now and then, maybe once a month, send me a link to something he thinks I might find interesting, but that's it.
It doesn't have to an affair like relationship and it isn't with us. He is just like a best friend, but different because we spend so much time together. And, yes, I do love him dearly, kind of like a brother, but nothing else.