The reason I'm not sure is less about the plain text, and more so the fact that the story has included "why did you use that word like that", "I don't understand that word" jokes because she is Russian before. The tone of the first mention of senpai, and the passing mention of a song recommendation both felt too casual even for Sasha.It is. There is no implication, she directly says they are sex friends with the senpai in the latest volume(3) We see the dude later again as well which makes MC act protective towards Sasha. Sasha and senpai appears to continue talking (he thanks her about a recent song recommendation when they come across him during their date) even after MC and her start dating btw.
Sorry, there is no universe where Sasha is not a slut.
Even if she has slept with senpai, its not really a deal breaker for me since she has signaled that their relationship now that it is formal it is exclusive. The Male MC also isn't a virgin anymore if the new Master+Sasha doujins are cannon.
The "it was casual sex and we weren't dating so its not cheating" is a very "mature adult in their 30s" mental state, even for the pro-sex Sasha considering how childlike and excitable she is throughout the rest of the pages.
If it ever gets brought up again we might get more direct confrontation of the issue, if not, I'll still read it.