I don't know the details of this particular case and I doubt that the whole, unvarnished truth was ever brought out in a courtroom.

If it had been non-consensual sex....

The victim's grand jury testimony was provided under oath and makes clear that this rape was non-consensual every step of the way. As Kate Harding wrote in Slate,

Roman Polanski instructed her to get into a jacuzzi naked, refused to take her home when she begged to go, began kissing her even though she said no and asked him to stop; performed cunnilingus on her as she said no and asked him to stop; put his penis in her vagina as she said no and asked him to stop; asked if he could penetrate her anally, to which she replied, "No," then went ahead and did it anyway, until he had an orgasm.

Polanski committed an ugly, violent act upon a child. The DA cut him a generous plea deal so as to spare the victim from having to testify at trial, but Polanski fled because he feared going to jail. His supporters then tried to smear the girl as a Lolita in the press.

There is a systemic interest here in prosecuting Polanski that goes beyond the victim's wishes. Allowing a criminal to escape sentence simply because he ran away long enough, or because he is a celebrity, rewards lawless conduct and serves no principle of justice.

Salon article: <link>

Grand jury testimony: <link>