Something I feel that the description fails to mention is that netorare intentionally arouses jealousy. From the description it just seems like Bob has to let Alice cheat on her or Alice just cheated.
But it's worse than that. Think of it like this.
Alice is Bob's first and only girlfriend
They've been together since high school and live together
They spend days just telling each other how much they love each other
Then Alice goes missing
Bob is completely distraught and looks for her.
He hadn't seen or heard from Alice for two weeks until he finally saw her just walking down the street.
Bob tells Alice how scared he was but Alice just responds with a "Oh look, it's you."
Alice takes Bob to her new home
She also describes how she never really enjoyed being with Bob at all; like it was too intense for her
Then HE shows up.
A fat, ugly, dirty looking man.
(Bonus points for interracial because that can really rile people up)
Alice has been living with this guy since she left Bob
Alice then proceeds to have sex with this man right in front of Bob all the while talking about how he's a much better screw then Bob could ever be
And well... Bob does nothing. He just watches, possibly even masturbates while watching.
This is a very common NTR plot. If you notice it has quite a bit of focus on the set up. Personally NTR has always left me with a feeling of tragedy, a desire for retribution, and frustration for there usually being none. Just thought I'd put my two cents in regarding the matter.