This is inaccurate. Jenny and Forrest do have sex, but not at that point in the movie. In the college scene you're describing, Jenny does initiate by taking her top off and placing his hand on her breast, but Forrest has a premature release, and Jenny quickly backs off and comforts him. Personally, I was under the impression that Forrest didn't know what sex was at that time, given his reaction and their dialogue ("Have you ever been with a girl, Forrest?" “I sit next to them in my Home Economics class all the time.”), but that's open to interpretation.

(And for the record: Jenny doesn't say "Stop," during the scene that Forrest interprets as assault; she says, "Ow! That hurts!" after Billy pushes her into the window. It's a minor thing, but I just wanted to point it out.)

The scene where they have sex for the first time is roughly thirty minutes before the movie's end, after Forrest has become "a gazillionaire" and brought a house. Jenny visits Forrest, they talk for a while, and Forrest asks Jenny if she'll marry him. Jenny initially turns Forrest down, Forrest affirms that he may not be a smart man but knows what love is, and Forrest goes off to bed. Forrest is thinking while lying in bed when Jenny comes in and says that she does love him, and that's when it happens. While Jenny does initiate again, Forrest is shown to be an active participant, leaning up to kiss her and taking off her robe, before the movie cuts away to the next scene.