Because Tybalt told her he was despite having no proof and she, for some foolish reason, takes Tybalt at his word without even talking to the MC first. She then has to stop spending time with the MC as part of Tybalt's conditions for not pressing (spurious) charges, and the MC finds out about this if he goes on the date with her in Ep 6 (where he also foolishly admits to being in the mansion and part of the sprinkler incident).
On the non-date path he won't know why she's avoiding him, but in either case he's not likely to accept being frozen out like that (he even declares as much on the date), and he'll undoubtedly do something in order to spend time Jill and this be will such a big deal for her that she give in to her desires to have sex with him, despite her virginal trepidation.