If I may, as the hubby in question:
We're all mono people — we're accustomed to one set of rules and social norms that we've all adhered to for all of our lives. That being said, if you can understand that the following intellectual and emotional positions emerge from a place firmly established with those set of starting assumptions, it may make this a little clearer. Hopefully.
The sexual experience bothered me because I explicitly exited the marriage because she didn't want sex. And understand, my primary motivation for exit wasn't that I had to have sex — it was that I didn't want her to have to have sex. It wasn't that she didn't want sex with me, it was that she didn't want sex.
She says her action was a combination of loneliness, nostalgia, alcohol and also pure intellectual curiosity which reinforced in her mind that her asexuality was universal. All of those things are independent of me with the exception of loneliness — so I don't feel any shame in owning my part of that situation that she may not view negatively, but that certainly leaves a bad taste in my mouth.
To be fair, the aftermath has clearly been viewed negatively by her — both in the manner that it has complicated a friendship that she valued and was a situation that created a temporary rift between us as I worked through my valid emotions while still respecting the intellectual insights she gained.
So, let me go ahead and throw out something I've been struggling with. The idea that I can have intimate emotional and physical relationships with multiple people and yet would not be comfortable with those people having intimate emotional and physical relationships with other people is something that's real and deep rooted. I'm just not.
I understand the imbalance and I loathe imbalance, but I feel like I'm in this situation because of a set of specific circumstances and coincidence buttressed by my sense of ethics and responsibility to my previous partner and my current girlfriend. The idea of monogamy is in direct conflict with my desire to love, cherish and respect the two people I want to.
tl;dnr = it's freaking messy and confusing and I'm doing the best I can, flawed creature that I am.
That felt good. Glad to have a place where I can talk like that...