This is an amazing piece of writing. If psychology detail of characters is your kink, you will love this novel.
Like many readers, I was a bit frustrated by flashbacks though they did seem to be of value - not to mention there was some really beautiful writing. You get to see that the Edward had a mentally ill mother which would explain his anger (other examples of violent display of which are also visible in flashbacks) and you get to see how Florance is conflict averse (she would leave her house in order to avoid any communication with the vaguest hint of conflict) so it makes sense that she kept delaying most awkward conversation of her life. Sex has something of aggressiveness inside it, something of our animal nature which frightens her who seems to have none of that aggression to herself.
Moreover, sex needs a second nature, away from normal social nature. The couple who has known each other for so long are embarrassed to show this aspect of their nature (it is to avoid this awkwardness that God invented role-playing games and foreplay).
And there is that love thingy ... you homo-sapiens, ever since Christ believe that love is a solution to all problems - your Harry Potter, Doctor Who etc don't help. If you strip it of all romanticism, love is just hormones gone crazy; often it creates more problems than it solves. It is the willingness to always see this emotion in rosy light people argue whether Wuthering Heights was a love story. You might, like Othello, love too much but not well. You might like Snape, love and still be a sadist.
The couple in the story love each other, and they both want to be best for each other. But they are at loss for the words that could describe their feelings. They keep on returning to the usual 'I love you's. But that is not enough, the frustration builds upon disappointing each other which can't be phrased either. Since the other person is also failing to communicate you feel cheated. And bottled up frustration, given enough time, always comes out as anger. I think it is telling that their divorce was filed on grounds of 'no communication':
Since your anger is for reasons that can't be spelled, it comes out on wrong things.
The parts that describe the argument, in the end, are simply brilliantly written.
"She knew he realised he had gone too far with his word, and now he was trapped with it. As she turned her back on him, she was conscious of play-acting, of being tactical in a way she had always despised in her more demonstrative girlfriends. She was tiring of the conversation. Even the best outcome would only return her to more of the same silent manoeuvrings."
Ian McEwan (On Chesil Beach)
Such beautiful descriptive passages, I am envious:
"This is how the entire course of a life can be changed–by doing nothing. On Chesil Beach he could have called out to Florence, he could have gone after her. He did not know, or would not have cared to know, that as she ran away from him, certain in her distress that she was about to lose him, she had never loved him more, or more hopelessly, and that the sound of his voice would have been a deliverance, and she would have turned back. Instead, he stood in cold and righteous silence in the summer’s dusk, watching her hurry along the shore, the sound of her difficult progress lost to the breaking of small waves, until she was a blurred, receding point against the immense straight road of shingle gleaming in the pallid light."
Ian McEwan (On Chesil Beach)