She was a virgin, technically. He did rape her, just in her ass, along with some other shit. So she was a virgin when she had sex with the MC for the first time. Was it necessary to include, probably not, but it does go a long ways in helping to explain why she hates him so much, why she wants to leave, and what she does to him later in the story. It also demonstrates that she's pretty strong mentally to have dealt with it and found a solution to get him away from her.
In all seriousness though, if you get squeamish about this, the game isn't for you, because some shit happens in this game.