Let's be clear: Invisible Man does not have overt sex scenes or over-emphasis on sexuality. It does have non-graphic descriptions of sexual situations. It's a matter of opinion whether they advance the story or contribute to it, but you can always have a conversation with your students about the necessity of sex in this novel.
Ralph Ellison was a thoughtful writer, so he wouldn't have wasted time on sexual matters if he didn't think it was part of the narrator's journey. The idea that Sybil has a desire to be raped by a black man is uncomfortable, but important, so this shouldn't be ignored in your conversations about the novel. Sybil's fantasy is tied up in the themes of gender, sexuality, race, and power, so situate the conversation here to keep the focus on what matters. Ask students to consider what new understandings the narrator comes to when he realizes what Sybil really wants.