Good thing NTR isn't predicated on cheating. Or being a love interest at all, for that matter. It's jealousy over losing someone to someone else, not even in a sexual or romantic context.
So yes, it is NTR.
I'll give you the credit of explaining it though Mike's perspective, since he, as his own character, is plotting on Sasha along with the other girls. So form the writer's perspective and intention, sure.
But as for playing the game, where everyone has their different intentions, purposes and point-of-view, including not caring about Sasha, or from my perspective specifically, the
blowjob happening several events before Mike and Sasha became romantically and/or sexually involved with each other, thus me feeling no jealousy over losing someone who I don't have at the moment, to use your description of NTR, I still think it's debatable due to the character dissonance between how Mike is written to feel compared to how someone playing the game might actually feel.
And while I'm at it, it's a similar thing with Lexi: Is Mike in love with the prostitute? Or are you, the player, in love with the prostitute? From my perspective: She's a prostitute. It's her job. In real life, the sex worker isn't in love with you just because you bought her used panties for $100 though OnlyFans, and you're not getting cucked when she decides to make a sex tape with her actual boyfriend.
No arguments with
Bree, since that can happen after forming an established relationship with her which requires the player's intention/motivation to do so.