Also sex is a massive part of love and relationships, especially your first love. I think the focus is just right.
Of course it's true of lots of our teenage/student relationships, but what is true in real life often simply doesn't work in fiction/TV. There's a lot of sex in the novel, but it's seldom detailed or written in long descriptive paragraphs, because SR doesn't do much description.
The first time they have sex isn't actually described at all. Marianne arrives at Connell's house, he opens the door and 'looks over her shoulder to make sure no one has seen her arrive'. Then the chapter ends, and the next one begins, one month later, with 'They're talking about their college applications.'
Obviously, you can't do that in film or TV, or not easily, but for me, it does change the emphasis and makes the TV version (still only seen the first two episodes, though) a bit duller than the novel because it slows it down.
And there's virtually no description of either of their bodies in the novel, other than that we know Marianne is thin and Connell is fairly muscular. We only know Marianne is small-breasted because the bullies say it. Again, though, you have to show something on TV, and you have to actually cast actors, so it changes the emphasis. SR chooses not to describe things, but TV has to.