Yes, a rape occurs in this tale. But the tale diverts its gaze from the actual event, simply narrating that it happened. We do get the potential for sex on the knight's wedding night, but he's so turned off by his bride that nothing happens. That is, nothing happens until she becomes a young, beautiful damsel, at which point "a thousand times a-rewe he gan hir kisse" (1260), but that's it. Which is why it's PG-13.