I think she had simply fallen out of love with him. I'm not sure how much the other guy really love his wife or whether it was 95% guilt! Certainly, he seemed tied in knots whichever woman he was with. However, I think that he tried to get to dislike his wife by setting her up and hoping she would do the dirty on him. When she did, but confessed all and told him she loved him, he couldn't bring himself to go through with it. I think he was also finding it a little sordid - remember the seen when he bikes over to his lover and her husband is asleep in bed and they have sex against the kitchen wall. He rides off immediately afterwards, somewhat disgusted with the whole thing. I don't know about you, but I came out of the cinema emotionally exhausted! Also, did you recognise the dark haired wife (forgotten the names now) - she was the young blond actress who was the girlfriend of the lead character in Blue Velvet - the one whose father was a policeman.