I scoured the doujin to see what the author usually draws and how the stories go so here come the nhentai numbers
500324 pages 6, 7 and 8
499822 pages 103 (author talks about IF and NTR routes) this whole upload seems to be a "Futoshi route" with side stories and/or alternative routes, I mean, first she says she speaks Russian because his father doesn't understand Japanese then later says her father taught her Japanese lol, also says her parents separated for the third time and mom brought her to Japan, page 220 talks about jealousy sex, page 285 spoilers at the bottom, page 315 jealousy sex Fake? Maybe like chapter 71/72?
492124 page 5 MC is not his boyfriend yet, they've had sex multiple times tho, no idea where in the serialized timeline this would go.
544195 page 9, "if I ever get a boyfriend I would never cheat on him" but if he agrees it's not cheating I guess page 27 this Manga goes before Futoshi and states that no cheating after Futoshi.
TLDR: Haguhagu seems of the mind that Sasha doesn't cheat once she has a formal boyfriend and has drawn MC stating that he wants Sasha for himself.
Of course he still draws the "IF routes" or 【IFルート】 which seem to be the serialized universe NTR (or maybe these came first the he decided to create the serialized universe, I don't know lol)
So at the end of the day it's up in the air but given how MC has been shown posesive I'd say it's OK to keep reading this until MC does a 180 and is perfectly fine with NTR or there's actual sex implied after Sasha and MC becoming a couple if ever. Chapter 72 will be important lol.