There is the parallel with the undercover policeman, but there's also the bit where she was penetrated by a foreign object that she did not agree to. If you agree to your partner sticking their fingers in you, but they actual use a bottle or their penis, or a vibrator then that's not consensual sex. And if you agree to them putting their penis in you but they actually put something else in you instead, then that's not consensual sex either.
It does all sound far fetched for what we might consider "normal" lives. But a lot of sex crimes are perpetrated against vulnerable adults who for a variety of reasons are more trusting and more naive than most people would expect. They are particularly vulnerable because in our culture, so many people still seem to think that naivety makes the victim somehow culpable and it's hard for them to get protection and justice after the abuse.