Exactly how steamy is this story?
PG-13
A pretty big chunk of Flowers for Algernon is dedicated to Charlie's desire to have sex, inability to have sex, inability to have sex without emotional attachments, and—finally—his ability to have meaningful sex. Whew—that's a mouthful. Sex is something Charlie can't figure out intellectually, so he often sees the old Charlie hovering around during the act.
Here's the general idea: Charlie doesn't know how he should behave to show his romantic interest toward Alice, so he chokes up on their first date (11.82). Things fizzle out between them, and he meets the extremely sensual Fay when he moves to New York. The first time he tries to have sex with Fay, it's a grade-A disaster—think complete reversion back to old Charlie and a total inability to perform (14.195). When he finally manages to successfully have sex with her, he has an out-of-body experience where the old Charlie watches from afar (14.210).
When Charlie finally gets it on with Alice, it's with a greater awareness of how sex is an act of giving. He tells us that "this time it was more than sex, more than using a woman's body" (17.293). It has less to do with intelligence, and more to do with genuinely caring for Alice, and we're happy for Charlie when he finally has this experience.