A lot of stuff on the slut route doesn't make any sense. Like when she tries to interact with her schoolmates at school and the community centre dance, most of them act like she's got the plague. But if she meets them elsewhere, they act totally normally, as if she's not actually an outcast. For instance, she can meet with Lazar on the sport's field on Friday afternoon and he openly flirts with her, yet a few hours later at the dance, he tells her to keep away from him.
In certain parts of the game, allowances for her being a slut just haven't been made.
What you mention is even more ludicrous if you make a June start, because you've had 3 months before the school term starts, where Sveta has potentially been slutting it up non-stop. She could have been whoring herself out in the City most nights, or at the local Gas Station or even the Brothel just outside Pavlovsk. Not to mention having sex with multiple randoms, Dan, Vitek and Vasily, Olu, Kolka, Anya, Vladimir, Tanya, Kendra, Djbril and his friends and a whole host of other characters. By that stage she can't walk anywhere in Pavlovsk without someone calling her a whore or a slut, and openly approaching her for sex. Like you say, it doesn't make any sense that she would be ashamed in any way. If she felt any significant level of shame, she wouldn't be doing what she was doing.