In the bathroom scene, the dialogue Emma has two dialogues, one he showed you and the other if you did not share it, where she tells him that she still loves him and that she is satisfied that she can do it with him from time to time, the scene ends when a guy approached her and she told him to fuck
First that bathroom is a "glory hole"
in any case Tony didn't ask Emma about generic cheating as you previously said
and i'm on the path were the MC is an happy-(alpha)-cuckold and he loves share all his girlfriends with strangers.
but the cock sticking out of the hole is not from a complete stranger.... Tony met him earlier in the pub in the following scene
So Tony asked Emma to suck the stranger cock in the GLORY HOLE (Bathroom)
Why doing the scene in a glory hole bath and choose to waste this great opportunity of sharing Emma?? (she told him in the same scene just some minute before that she would like to be shared with Martin) .. Emma loves to be shared!!! There is no logic in don't ask Emma to suck the BBC in the hole!
Tony explicitly asked Emma why she left him for his father... and this is also related to your post were you said:
Martín does not steal from Emma but seduces her as he did with Anne, the difference is that the mc broke up with Emma and he doesn't break up with Anne, Emma in a dialogue says that she thought she could maintain both relationships, neither of them feel anything for Martín only physical attraction and in the case of Emma also his lifestyle
As you can see here you are just dream on what was never written in the story
Tony still think in the scene in the bathroom that Emma was stolen by his father for cock size reasons and only after talking with her in the CH 12 ends
only after that question he reailzed that he was wrong thinking about Emma was stolen by Martin.
This story has dynamics and the MC can change his mind during the story progression.
at the CH 7 the MC was thinking that Emma was stolen by his father for cock size reasons
but in CH 13 after talking with Emma he realized the truth about her and his father! She would like to fuck with both Tony And Martin as well.
( same as Anne
... there is not such a differences in the girls Tony wants as girlfriends)
Anne was the only one that stoped him for 5 years before starting sharing her
hope this helps to understand the story logic during progressions and time passing of this masterpiece!