Turns out that the teacher who was a really dedicated one also had a secret life as a total nymphomaniac though she was scrupulous in keeping her lives separate. But the wall of separation is crumbling as one of her students finds some nude photographs on the internet. It does beg the question as to how secret your life can be when you let all hang out on the internet.
In the end ADA Cabot just can't get a murder 2 indictment of Nolen who is a real low life because the Grand Jury can't get past the victim's sex life. The fact that he's already stalked another woman doesn't weigh enough with the grand jury to overcome their biases.
It's sad, but true that in criminal justice the victim often goes on trial in the defense's case. This episode illustrates that quite well.