Your right, Frank and Mary have talked about it beforehand at least on the faithful path.
Anyhow, a choice afterwards would change the perception of this scene and remove the railroading into a hotwife light path. Also you may set a flag for possible future content of that kind, lets say to define how far mary may go. From that point of view - would it have hurt to have a small split in the path?
To be very honest, it's really painful to run 3 paths at a time, that too with equal progression, the amount of time that goes for script writing, renders, animations, and other stuff is very time-consuming. Adding another split is just an additional burden.
If Fred's reaction mattered (in the case of multiple protagonists), I'd understand. But since it's Fred's POV and even if she had an orgy in front of Fred and he's like I don't like it, we can't get a reaction out of Mary for Fred's response, like being apologetic or regretful etc.. They are already past that point where she is more open to sexual stuff, if this were Chapter 1 or 2, I'd agree. Chapter 5 is like things have changed drastically. Since this game is about Fred & Mary, having them split is a climax scene altogether. We are yet to end the blackmail route and a few other storylines. Having them split and go their ways is not feasible for the story at his point.
Rest assured, the game will have at least 2 different endings in Chapter 6 (Divorce is a possibility)