I've been in the OPs shoes once.
I met him when I was 29 and had been through a string of bad boys and along came this lovely man who treated me like gold. He was going through a divorce when we met and he was quite cut up about it. The wife had fallen in love with someone else and was leaving him, so he was a sympathetic character in my eyes.
After I was well and truly in love with him and he'd made me feel like I was the love of his life, he confessed he had not been faithful to DW.
In fact, for 15 years together he had cheated (always one night stands, sometimes with prossies, sometimes with girls he knew) and it had happenned once a year or once every two years.
I was horrified by the confession and almost broke it off but he told me all the same things. He felt awful. He had loved her but she didn't like sex and did not give affection. He felt rejected and was lonely. It would never happen with me because our sex life was out of this world and I understood him like she never had.
I lapped it up. It made me feel special, as if I made him a better man.
And you know what? He never did cheat on me. He was faithful for years, devoted, kind, loving and every day I was with him I felt wonderful.
Thing is though, 5 years into it, I got ill. I was stressed out and developed and anxiety disorder that meant things changed and I needed support. Oh he stayed faithful and devoted but he just didn't have what it took to love someone when they were no longer fullfilling his needs and he left me.
Oh he was as devastated by the split as me (perhaps more so) nd he actually never met anyone else; but such was his character that he did not make a good partner.
He was a person who would lie rather than be honest. He kept telling me my illnes was not a problem and he only loved me more, instead of discussing how it was affecting him.
He was a person who cared really about himself over others, even those he loved.
He was a person who was just not capable of weathering the tough times.
Over time, I came to look at things from his exDW's perspective and I reckon he probably did like dirty sex more than she did - but then why the fuck did he marry her? How can he blame her for him pretending to be someone he wasn't in order to be with her.
Sometimes you just don;t see clearly when you are in it, but nowadays I could not and would not be with anyone capable of lying to their DW, incapable of moving out and divorcing before sleeping with other people or someone who could compartmentalise to this level.
Like your ex, she didn;t have a clue...she STILL doesn't have a clue any of this ever happenned and he robbed her of her life in many ways.
I have no doubt he absolutely loved her, and me, to the best of his ability. I reckon he probably loved me more than he ever has or ever will love a person -but it didn't mean he made a good partner.