Don't really see the NTR aspect. Sasha isn't now in and hasn't ever been in anything remotely like a committed relationship in any universe or timeline. She's a promiscuous girl and has been from an extremely young age and only now is beginning to experience the difference between an erotic life and the feeling of eros. What she does about it going forward (and what her "master" does in response and is willing to put up with) is what this story is and always has been about. If they start actually dating and it's not at least somewhat an open relationship (unlikely given this is, y'know, Sasha) and she's still seeing Senpai or Fat Bastard or whoever...well, then, that's another matter entirely. If she's sharing her past exploits with him or bringing in another partner with consent, that's its own issue as well, but still not NTR.