The BBC Trust's editorial standards committee said the episode, screened last April, was not suitable for its time slot. It drew 632 complaints.
The committee's report said: "Viewers were left in no doubt that an act of sexual violence was being carried out... this level of explicitness of sexual violence would have exceeded viewers' expectations of this series."
The watchdog said the content had greater impact as it was screened later than expected due to an earlier programme over-running.
It apologised to complainants and those "taken unawares" by the scene.