I think the issue isn't the term... More so the fact of her being so close with a male colleague that they are throwing around the term "work husband". I do trust my wife; I trust her not to do that.

I have female colleagues and we have good relationships. We help each other, we get along great but I don't go cozying up to them to the point that we start referring to each other as "work spouse". Tbh I'm a faithful guy, but what gives me confidence in maintaining that isn't purely unwavering self-control or the power of love. People are weak, feelings shift from day to day. I'm no different. My marriage is precious to me, the trust and honesty I share with my spouse are precious to me.

So to protect that, I avoid situations that present a high probability of temptation to cheat, emotionally or otherwise. And having a "work wife" suggests itself as one of those situations. If I have another woman in my life who I share everything with, that I'm tempted to confide in when I'm unhappy with my wife, that feels sketch.

Due to the context of that situation, the shift from "close work confidant" to "woman I'm complaining about my wife to" is so minute as to be almost imperceptible, and that's exactly the danger. Because the latter is what I'd consider borderline emotional cheating, and you barely did anything different. You told her about your night last night, a fight you had. Big whoop. See the problem? Then maybe she starts unloading about your husband too. Maybe you're in a good place with your wife but want to empathize, so drudge up an old grievance and join in. I hate to invoke the slippery slope argument but it certainly has been for a LOT of people, and for a reason.

And if you're gonna say "ha! See, you don't trust yourself around women, you're projecting!" Stop. I trust myself around women because I have boundaries I don't cross that are far short of actual cheating.

Just like I trust myself to get to work on time, not through strength of will, but because I bought an alarm clock.