'Pure' only in the sense, that she wasn't already a slut. Because she'd already had a fair amount of sexual experience by then, with quite a few different people. So you could hardly describe her as being innocent or sexually naive. Andrew had this idea in his head, that Anna was like that. But he had no idea of the naughty things she'd already done. We do, because we've played all the short novels. So at the start of the game, we're viewing Anna in the same way he does, and it appears that she's a 'pure' girl, and the way Anna views her previous actions through blinkers, only seems to encourage this view of her, as being this innocent, naive country girl, suddenly thrust into the perversions of the big city. But the reality was Anna had been flirting with being a slut for a while, and in all likelihood she might have become one anyway, even if she never developed her condition? That just made it happen a lot quicker.
Because even without her condition, she was always going to be exposed to her horny, slutty sister, her equally slutty bff, her sleazy, perverted boss, and her randy old neighbour, who reminded her so much of her grandfather. Not to mention manipulative Ashley, her boyfriend's bad boy dad who she'd always had a secret crush on, and all the other crazy, sex mad folk she bumps into in Sun City The chances were already high, that at least one of these people would have pushed Anna down a dark path into infidelity. Because these types of scenarios tended to happen to her anyway, which is possibly evidence she has some natural prediliction towards slutty behaviour.
So maybe all the accident, and the ensuing 'condition' does, is accelerate what would have happened anyway?