I'm not inclined to assume
too much about what isn't actually stated in the text. As a corollary, I'm also happy to accept unanswered questions. We don't know what we don't know.
Chapter 4 of the main story is obviously designed to suggest and dovetail with the events of the non-canon R18 chapter 4.5. But taken on its own, the in-canon chapter provides no unambiguous evidence of sexual contact. Same goes for the relationship of chapter 2 to 2.5.
As readers, we're offered a choice:
- We can accept the author's canon version as the complete "real story", in which many questions are unanswered and we don't yet know anything about the MC's sex life...
- Or we can incorporate the non-canon R18 events, which fills in some of the narrative gaps and in which the MC both has consensual sex (ch4.5) and is raped (ch2.5).
It's ultimately a matter of personal preference. I prefer to accept the main story as complete in itself, and to view the R18 chapters as "what if?" digressions. It seems that's what the author intended. You prefer to incorporate the R18 chapters into your own personal headcanon because doing so allows you hate the "slut" MC, which you seem to enjoy.
And that's fine. You do you.