Do you think Liam is the protagonist in the June story then? Or Henry, in Emma's story?
See also : Anna Karenina.
They are deuteragonists for sure, although their proximity to the events of the story make it hard to classically call them a protagonist. Then again, NTR stories typically disrupt the normal structure of a story because by all intents and purposes, the goal is to fail.
EDIT: In thinking a bit more on it, I think I might actually classify Henry as a protagonist, but at this point in time probably not Liam who is still the deuteragonist of their story.
I will say, it would likely be more accurate to change the description to mention the act being done to any primary character rather than just the protagonist. But like I said much earlier, overwhelmingly NTR stories are told from the perspective of the cuckold because that's the best way to convey the proper emotional ideas of loss and jealousy upon the reader/player, and so "protagonist" is right 95% of the time.
