Well, as I said, the vast majority of harem/vanilla players don't want NTR or NTS; they dislike the idea of another man being with the protagonist's womens. Even if the content is optional, you should at least warn people that your game includes NTR, since it currently only has the "romance" tag—and for those looking for one of these genres, romance and NTR are almost always polar opposites.
I'm just saying that if the game's only NTR element is a "Game Over" screen, and you're aiming for a harem/vanilla audience, you'll likely drive most of them away. As I mentioned, the fact that it's "optional" doesn't matter to them; given the sheer number of games out there today, it's easier to just move on to another game than to waste time on one containing things you dislike. At the same time, you won't attract much of an NTR audience either due to the lack of content—since, as you said, it's limited to just a "Game Over" during fights and doesn't amount to much substance.
I'm just saying you should add that the game contains NTR so you don't mislead future players, since it's a genre hated by many.