(Very) generally speaking, people who dislike NTR are not interested in seeing scenes with love interests (current OR future) interacting intimately with people other than the MC, period.
If a character is intended as a love interest in a game, players who do not like NTR do consider that character to effectively belong to the MC from the start of the game, since the goal is expected to be the establishment of the relationship between the MC and the love interest(s). This is a particularly common view about incest-based games, where the default assumption is "male MC + female family members = future incest with all of them".
A notable exception is when one of the female family members is in a relationship where only non-intimate scenes with her husband/boyfriend/whatever are shown, and the MC steals her away from her existing partner (netori, which people who dislike NTR usually have no problem with and do not commonly include under the "NTR" umbrella term), frequently because the existing partner is neglectful, abusive, or deceptive.
I haven't seen the scene in question, so I'm making no value judgements here about the scene itself, but "it's not NTR, because the LI wasn't with the MC yet" has been used as a defense for forcing content like I described above on players who don't like it for years. The assumed future relationship applies when evaluating whether the scenes will be acceptable to people who dislike NTR.