These are my thoughts on what you've said.
Regarding the email itself, it seems like a matter of perspective. In a relationship that has never been touched by infidelity, I see no problem with this email. Things such as job interviews have social elements. Employers hire people both in terms of skills/experience and also by how much they are seen as "a good fit" for the company and the team. Small talk and conversation are part of this process. It being Halloween, there were similar conversations between co-workers at my workplace. (Do you typically get a lot of trick-or-treators? What did your kids dress as? Do you have a curfew in your area?) I've actually learned to "lead" conversations with a quick comment about my wife or kids when speaking to new people, especially women, because I feel like it sends a message and sets a soft boundary from the get-go.
That having been said however, the "real answer" in my mind is simply that your email is not the issue here. The issue is the effect it had on your wife. This was by far one of the more difficult things for me to get through my thick, wayward skull. Once I got it however, it stuck. Look, if an apple falls out of a tree and bonks you in the head, it is useless to blame the tree. The tree had no ill intentions. But in reality, that doesn't matter. Your head still hurts. The apple still came from the tree. If the apple cracked your skull, then you may still have a lifetime of healing or a TBI. Regardless of blame or intentions, the damage was still done. That's what happened with your email as well. Intentions be damned, damage was done.
Your wife has been hurt, lied to, and betrayed... so many times. For so many years. I imagine at this point it would be triggery for her to see you interact with another woman in any way at all. There could be a shampoo commercial on TV, and she might wonder why you chose to look at the TV at the exact moment? Why did you tip the waitress so well? Why did the lady walking her dog wave on the way past you? I've read many, many posts by BS's who say that this type of "constant vigilance" is what drives them crazy. What WS's don't realize is that this is typically "all day, every day" especially for BS's in their first few years after discovery. It's why they say their WS's "don't get it", because the WS has no idea how tortured the BS is, regardless if they voice those thoughts or not.
You can't really avoid triggers 100%, no one can. But what tends to help the most is to see the WS make the effort, and when effort fails, to see understanding and empathy. What she needed to hear from you was that you understand why she was upset and that she has every reason to think that way, and that you are also upset, not only about what you wrote, but moreover, for not accurately anticipating how it would make her feel, and the fact that she has to feel that way at all in the first place. She needed to hear that you were sorry for putting her into this position in the first place (owning it) and showing some understanding of how deeply unfair and painful this is to her. Not just now, but in the past as well.
This incident is part of the learning process. Rather than beat yourself up over it, use it to learn from. Go over how you think you "should have" handled it, and do your best to make plans or create tools to prevent it in the future.
One last thing. "Fair" is fuzzy word. A lot of WS's think about what's "fair" to them, and to their spouses. Is it fair that you have to watch every little word when speaking to a female co-worker? No, it's not. Was it fair that you cheated on your wife? No, it wasn't. She's stuck dealing her unfair experience, and the best way you can help support her is to also share the burden and accept that some things in your life will be less fair than for someone in a stable marriage. It's the price we must pay. The good news is, two people dealing with that sense of unfairness together, can help bring them together sometimes. It becomes less of "my story and your story" and is instead "Our story, this is what WE survived".
If you have empathy, then you never need to get defensive.