This is my guess and the author is dragging out the reveal because he knows people will drop it once it comes to light.
That would make sense with her leaving town and such.
But it actually makes the whole series worse for various reasons:
1 - It makes the fact she was worrying about the masturbation thing to feel ridiculous because of how obviously worse the other secret would be like.
2 - It makes the whole thing be ridiculous regardless because MC already thought she was an experienced girl, both because of those flashbacks of her having a boyfriend and male friends, her flirty personality in the present, and the previous misunderstanding of her having sex almost all the time in her room. And the fact is that MC gave a fuck about her possibly being a slutty girl, so confirming it wouldn't change s thing.
3 - Despite the sus scenes here and there, the author has given few real hints that she is a slut, it has been mostly things left to interpretation which caused that: if she is a slut people will be mad. And if she isn't a slut people will also be mad (because the author wasted our time).