The sex in Sula isn't just there for shock value. Attitudes toward sex give us vital clues about many of the characters in the novel and about what one generation passes on to another. Things can also get confused and disrupted when people don't operate from the same understanding of sex. For some, it's casual and easy, but for others it carries much greater weight. When these two attitudes collide, major misunderstandings can ensue.
Hannah's casual attitude toward sex ultimately leads to the break between Nel and Sula. She has passed it on to her daughter, who then fails to see how it might hurt those around her.
We should blame the cheating husbands in the novel, not the women who sleep with them. The men have a responsibility to their wives.