Sexual precociousness in underage girls is literally a sign of sexual abuse.
It's manga; precociousness isn't a sign of anything.
Chapter 9, page 8 has her literally say, out loud, that he taught her about "Choking play". There are a number of issues there. First, she's giving everyone the impression she's sexually active (which you say she wants to avoid), and second, that she didn't know about choking. Either she's lying about the choking thing for no reason (since she's been choked for YEARS now during sex) and ruining her OOC plan to not be seen as easy- or she just learned about it, and
this happened after that chapter (It's not titled "IF", so it actually happened- long enough for him to comment on the choking, and it's NTR).
You can't even say she didn't realize the implication: The other guy spelled out what calling him "seggs master" implies in the page before (Not that she'd need him to if she's sexually active: He'd know less than her).
We never see that sex is on the table, even in her private moments at home, and with friends. She has free time in Chapter 4, and none of her ideas include texting anyone, picking anyone up, or anything. It's all mundane stuff like exercise and eating out.
She clearly was keeping her relationship with the senpai secret, as shown in 30.5 (while he was introduced earlier), even she knows becoming known for being easy would be bad for her; moreso than being known as an ecchi otaku.
I'm pretty sure we're talking about Sanada Senpai. The one her friends said she turned down, then flaked on in ch. 18-19. He gets retconned into a sex friend too. Slut Sasha would hang out with him, then tell her friends the details. They wouldn't be asking why she turned him down. Case in point: She told MC what was happening here with even less prompting than she got the last couple of times.
To boot, after her talk with her friends, she's literally telling herself she's turned him down because she likes MC more (something she directly contradicts later, for the record). She doesn't have to lie or keep secrets from herself. If she accepted and kept it a secret, who she likes more wouldn't be relevant to her.
The author is obviously just trying to explain away why she broke character for two volumes.