I'm glad you're communicating more HV and I just wanted to pick up on two points.
Loves2Walk gives brilliant advice above - and if you're reading Not Just Friends, you will see that people having affairs are rarely not "getting" enough of anything; rather they are not "giving" enough. But it is circular in nature.
One stops giving, which leads to the other not giving either. In all the turmoil, it is difficult to see who stopped giving first. However, in the classic "distancing phase" that needs to happen in order to justify an affair of any sort, it always starts with the betrayer. Put simply, the betrayer behaves like such an arse that they get a response behaviour of less affection and warmth. Even subconsciously, the betrayer can then tell himself that "I wouldn't be doing this if my wife were more affectionate..."
It is deeply manipulative behaviour and needs challenging, along with everything else. Your H might not even have been aware that he was doing this, but in a marriage that has always been happy, a betrayer needs to create a gap to allow the affair to start or continue. At some level he realises that he just hasn't got the tired and hackneyed affair justifications of "my wife doesn't understand me, want sex etc." so he behaves so badly that pretty soon, she doesn't understand him or want sex as much as before.
Finally, I completely understand your comparisons with friends' marriages, but believe me, all is not as it seems. My H has always been the "adoring" sort and since we have told very few people about his affair, I'm sure they'd look at me and think "her H would never do that".
This is the downside to not telling people what we are going through and is something I wrestle with all the time. However we have had a few disappointments in terms of understanding from a couple of people and in those cases, telling did more harm than good.
However, I've decided that if any of my friends were brave enough to tell me that something similar had happened to them, I would share my story. The best empathy I got was from someone who had been through something similar. Honestly, HV, from the outside looking in, you very often cannot tell who would do this.