FMC is like prostitution not ntr .
Despite having a preference, it still is ntr, just a different flavor.
edit: Read a bit on the dlsite page (machine translated) and it said this
"Certain events progress from the husband's or son's point of view.
Meanwhile, Selenia is doing something she doesn't want her husband or son to see or know about.
*Even if you don't see the flashback from Serenia's point of view, it will not affect the progress of the story. "
As for how much of that is in the game, I dunno.