The reason why you get critic for this is, it's not a harem game, you are forced to choose one LI.
So most will make several saves fo the different LI's.
And yeah while the LI you reject isn't in a relationship with the MC and as such it is not technically ntr, the thing is, most still want that LI in anothe run and don't like to see ''future'' LI's banging a other character.
For many it will still feel like ntr, especially if this scenes are with other men (many are a bit more lenient if the scene is with a other women).
For me, i have no interest in lewd scenes which don't include the MC at all and find them just mostly wasted work, which could have gone into a scene with the MC and a LI.
If you just show that the LI's you couldn't pick move on with their lives, get a bf/gf that's fine for most...giving them sex scenes with others not so much.
