Ffs... Let's try this again.
If the player makes the choices that push her to it,
it's still not part of the main story at all because Chloe is not the focus of the game's story.
It is avoidable, by the player's choices.
It is optional, based on the player's choices.
Chloe is the "default" girlfriend that the game starts with, daughter of John (the game's main antagonist, not Beeman) and his wife Nicole. The main story of the game is centred around the MC seducing and getting into a secret relationship with Nicole, his own girlfrien'ds mother. It can be done while still being together with Chloe, or not. It can be done with Chloe cheating on the MC,
if the player has made the choices that push her toward that.
Do you understand the concept of player agency, and how it relates to choosing options that are presented in the game?
That's what this is. The words are quite literally, interchangeable here.
Well, you're wrong on both counts. Chloe is a secondary character, and yes avoidable and optional
mean the same thing in this context.
This. Context...
Your personal "feelings" about the words "optional" and "avoidable" don't factor into the reality of the situation. Many words have different meanings
based upon the context of their usage.

Where, oh where in anything I've written, did I ever say that it wasn't?

Play the game...
I've literally said how to avoid it
multiple times, particularly in my responses to
you.
Read what I've actually written, instead of focusing the existence of ntr at all.

No, it's not.
Those plot links relate to the teacher, Chloe, his family and younger days, and you see this aspect play out regardless of Chloe still being with the MC or not, cheating on the MC or not.
There's a shitload of spoilers throughout the thread, so if you want to know, either read them or...
Play. The. Game.