I think the different sex scenes are used to give the reader an insight into the characters. What Pecola's father does to her is awful and Morrison is not making excuses for him but she does make the issue more complex by having included the scene of Cholly's first sexual experience one that was dominated by the white hunters. In this way we can also see Cholly as a victim. This does not make up for his abuse of the daughter but it underlies the immense tragedy of the whole situation.
Through the various scenes which include sex, the reader is able to get insight into the characters as well as see the complexities of the issues of racism and ideal beauty through the different ways in which sex or sex appeal is portrayed.