This was hot, you guys can hate on it all you like, but this was really good.
This doesn't feel like NTR to me because Ayaka doesn't agonize and despair that her bestfriend and object of her love is in a relationship with another girl; instead she gets off to it. Kanako keeps her relationship a secret and Ayaka keeps her masturbation and fantasies about the relationship a secret and so both come to terms with their situation and remain friends.
I suspect that both know about each others secrets, even though that isn't stated, but that's just me. Something like that doesn't usually remain a secret for very long.