This sounds so familiar and so very sad.
Its not your fault you didn't have the desire he wanted / needed. There is no way you could have understood how he felt, and you did the best you could do. You could physically do what he wanted, but you could not have the desire that he really needed.
Its not his fault that he couldn't enjoy love without sex. He could not have understood how you felt. He cheated to try to get the thing that was missing from his life - something meaningless to you, and inseparable from love to him.
Having your friends think you have a wonderful life makes it worse. We are also the perfect couple. So friendly and affectionate (in a non-sexual way) in public. We go on romantic trips, enjoy our time together. No one could imagine that on our 25th anniversary, staying in an ancient palace on the grand canal in Venice, my wife spend the evening reading, and I spent it playing computer games.
I wonder how many couples are like this.
I don't think it's as familiar as one would believe. The drifting apart and having different priorities - yes, absolutely. That's all too common. When sex isn't important to one person, it's easy to completely forget about it entirely. However, they can be reminded that it's important, provided their interest in maintaining the relationship supercedes the interest in doing things they don't understand.
What I see in OpenMinded's story isn't simply a lack of desire, but more an inability to compute sexuality, thus leading to an unwillingness to play along any more. And that's fine. The inability to compute sexuality (as well as lack of sexual attraction) is what defines asexuality. Oddly enough, lack of desire does not. There are many asexuals who HAVE desire (mainly for physical stimulation).
Many people who identify as asexual don't understand sex in general, but they 'understand'
its importance and role in a relationship. But then there are a small percentage who either don't, or decide 'enough is enough, I can't do this any more', and take being single over the grind of being married and having to be somebody they're not - even if it's in a relatively small way. And that's okay.
But, it's difficult for us non-asexuals to fully understand the challenges associated with engaging in sexual acts, say, once a week, for the sake of an otherwise good and healthy relationship or marriage. And it's exceedingly difficult for us to understand why we might not be worth it. In my experience with an asexual wife, I understand that there's no physical attraction to me, or anybody else. That was not easy to comprehend at first (we all want to be physically desirable to our partners). But once I wrapped my head around that, things were okay. My wife does get something out of sex, so at the moment, it's worth it to her. I don't believe she forces herself to do anything she doesn't want to do. She does get physical pleasure from it, which she enjoys (but does not require the way most of us do). She does get closeness and intimacy (which she DOES require). And she gets a good relationship from it (which she also desires).
A couple of years ago, I had a post in which I compared sexual needs to (I think it was) bowling. Weird analogy, I know, but it's what popped in my head at the time. Now I can take or leave bowling, and could never bowl again in my life and I wouldn't think twice about it. But when I do it, I have fun.
That said, if I married someone who likes to bowl once or twice a week, I'm not sure how long I could keep that going, or how fun it would be after a year or two of that. That would be up to me. Whether the trade off would be worth it, I suppose. Maybe eventually I'd grow to hate it. Maybe it'd just become part of my life and I wouldn't think twice about it. Tuesday and Saturday becomes bowling night. Or I could go all-in and we'd join a league, my wife would be happy and it's a small sacrifice I'd make for her sake.
IMO, there's no harm in doing something that makes your partner happy, provided it doesn't make you UNhappy. Would I like to bowl twice a week? No, not particularly. But if it was important to my wife, I almost certainly would, and would make the absolute best of it. After all, it's time spent together.
On the flip side, I'd expect her to understand that bowling is not a high priority to me, and that she shouldn't expect me to view it the same way she does. I wouldn't want to be berated for throwing a gutter ball once in a while.