All true, but you're making assumptions. We only know about the MC, so all the accusations of her being a massive slut who's taken 'miles of dick' are based on her image and what some catty bitches say about her. I highly doubt she's a virgin outside of the MC or anything, but maybe we shouldn't be throwing stones considering what a massive man-whore the MC can be (depending on your playthrough)? She's more likely to catch an STD from him than vice versa.
edit: also, you DO know how 'burden of proof' works, right? You can't prove a negative. Of course I can't tell you for sure she hasn't screwed anyone else... I can't prove unicorns don't exist... doesn't mean that they do.