It's straight up the whole point of the title, as someone else in the comments called it, Schrödinger's Slut. At this point, I'm more interested if when the other girls show up, would they be in the same slut-or-not box like the main girl just to keep the whole thing rolling.
The MC and the readers are given a smoking gun (opened condom wrapper), no bullet (the actual condom), but shots were definitely heard (moaning). Sus as all heck for sure, but not what one would call proof beyond a reasonable doubt that some other dude is banging her. You'd also think he'd notice any guys entering or leaving her apartment if her moaning kept him up all night.
The raws have the tags いちゃラブ("flirty" love), ラブコメ(love comedy), and 尽くす系女子(devoted girl). If that's any indication of what's to come, there will be more fake-outs in the future.