I dont quite understand that mentality.
In a game like this, if you are not actively pursuing a LI, by story means, why would them shown in sexual acts with others in that playthru considered NTR?
Hell, what is NTR? My understanding of it is that it's an EXTREMELY specific niche of hentai anime in which husband is cuckolded specifically by the wife. How does it even translate into F95 userbase's understanding?
If anything, I find the fact that 'abandoned' LIs not getting a proper partner / ending more dissatisfying than seeing them in sexual acts with non MC characters.
there is this extended definition of NTR which includes any sexual act between a character in which the player or the MC might have an interest. it applies even when this LI would officially be in another relationship... strangely enough it does not apply to lesbian sex ever (I don't understand this either...)
staying in BADIK, we see Maya and Josy twice in intimacy without MC, once is a flashback, the second time is when Josy trains Maya for MC, so even if MC is not physically there he is still the centre of the scene. and we are still talking about two girls for whom it seems that the anti NTR troops have more tolerance.
personally i think you rarely see intimate scenes of the other characters for essentially 2 reasons
the most trivial one is the fact that players don't like them, I don't know if it's the majority of players or a particularly noisy minority
but the main reason in my opinion is "economical", they would be scenes to create in addition, not related to the main plot, they would be completely additional work for the developers. adding in the final some renderings with the other LIs is relatively simple, but to carry on their storylines, separated from that of MC, would be a huge amount of work, it would make the story more complete and also the conquest of the chosen LI more satisfying, but undoubtedly would increase a lot the workload of the DEV