Well it's an interesting debate you have started here, and I can see there are as many theories as there are affairs. Personally my view is that no two affairs are the same. Some people cheat because they can, and they enjoy it, others to escape something, many are serial cheats, many are not, some even leave a partner because they fall in love, properly, and the new union lasts. You can't define anything where human emotions are involved because emotions are by their very nature at times both illogical and irrational. Love, lust, loyalty.... strange concepts all of them.
You are a strong and intelligent woman, but sometimes you come across as being almost consumed by your husbands betrayal, perhaps even allowing yourself to be defined by it. Why let him infiltrate your thoughts like this? He is history. Gone. A weak man. Not worthy of another thought.
In a strange way I've felt connected to you on here because we went through the pain together. You are right, no bystander can even begin to understand what it feels like. You have to let go though, you have to invest your thoughts, energy, passion into something outside of this.
MN relationships board is a little dangerous. I'd been on here too much recently, and you know what, it set me back. I was doing so well, and then I found myself getting sucked back into doubts and obsessions, fuelled by stuff I was reading on here. I had to step away, find myself, my independence and my self confidence again. It's back. The rest, it's over.
I'm not a therapist, I haven't read fancy theories, I don't speak the current lingo, but I have walked a mile in your shoes. It hurts like hell, but it hurts a hell of a lot less when you close that door, open a new one, and start your life again.
Hugs