There's nothing graphic at all in The Heart Is a Lonely Hunter. In fact, it's downright vague even when Mick does the deed. Sexuality and the body are highlighted more than the act of sex itself, which makes sense in a novel focused so heavily on individuals.
But we're not going to sugar coat it: sex is a fact of life in this novel – just like illness and hunger and violence – and it's not something from which the novel shies away. We've got the Palace of Pleasure, Copeland's domestic saga, Biff's sexuality and impotence, Singer's feelings toward Antonapoulos, and Mick's blooming sexuality. We're just saying: bring along some chaperones.