Hey there!! Thanks for reaching out. I think a lot of this defining of avoidable, skippable, optional, etc. is just kinda semantics at this point.
I'm little confused, but I think I know what you mean. I'll try to simplify it. And I appreciate you not starting an NTR argument haha. I would meme it to oblivion.
So yes, if you choose to opt-out of NTR you will miss out and not see any content/scenes that directly reference any NTR scenario. They aren't "happening" in the background, they just won't take place in your game/save. So you obviously won't see that portion of the game, however large it is. The amount of NTR per update will vary, but it will never be the majority of the content. (except for maybe one chapter. Out of many chapters.....) Likely a scene or two with each update.
NTR isn't part of the main plot, so the content you'll be avoiding won't influence it much. But certain characters that are aimed at NTR will have less presence obviously if you are avoiding most of their content.
So let's use that "pseudo rival" as an example on a scene in thr game!
NTR OFF:
Rival character arrives at the player's home, greets one of your mothers, and enters the kitchen where your sister Amelia is making breakfast. She will ask for a piece of toast, which Amelia will hand her begrudgingly. Then the Rival makes their way to the living room with toast in hand and chats with Laura, the MC, over coffee. Amelia enters the living room with her own breakfast and eventually notices Laura sporting an erection. The rival will get up, say goodbye, and then leave. Amelia will then make her way over to Laura and offer a blowjob with her morning coffee.
NTR ON:
Rival character arrives at the player's home, greets one of your mothers, and enters the kitchen where your sister Amelia is making breakfast. She will surprise her with a slap to her ass and mention that Amelia is lucky that she is hungry for breakfast. The Rival will then go down and perform oral sex on Amelia as she cooks. When it ends, Amelia will mention that she actually eat something, and offers her a piece of toast. Then the Rival makes their way to the living room with toast in hand and chats with Laura, the MC, over coffee. Amelia enters the living room with her own breakfast and eventually notices Laura AND the Rival, sporting erections. Amelia will then make her way over to them and offer a blowjob to the both of them with their morning coffee. After that, the rival will get up, say goodbye, and then leave.
This is how any divergent stories are written.
Hopefully that clears it up for you.