For a lot of fans it isn't a particular fetish being canceled as much as the game being pulled in a direction that it was not headed. DMD was about corrupting D and her maturing as an individual. Now it seems like the story has become a series of mild uninteresting events.
It would be like getting tickets to the Super Bowl only to find out when you get there the game has been replaced with elementary school kick ball.
Oh yeah, I totally get that.
I just found it an interesting observation that an MMF with F,D,Martin could actually be addressed and rejected in the game while remaining consistent with the story development (we have no idea if Martin would be down for an MMF in any situation, let alone one with two blood relations).
I also completely agree that the main story arc of the game has always been about D's developing independence/agency. I've said before that I see her now as being as much of the protagonist as F is, and the story from this point forward should be highlighting that reality. The storyline of her continuing along her corruption path could be handled via a scene with Ryan while the Martin story still (based on events to date) could unfold in a few different ways. D could be the one to propose the MMF to F and Martin (a sign of her growing agency and corruption), and Martin could in theory reject the idea in a manner consistent with the story.
Exploring how D might process Martin's rejection of the MMF might be an interesting pathway. Would it lead to her thinking about how she's grown as an independent woman and what she wants for herself? Would she, after hearing her own mother's confession, reconsider whether being with F long term is the right thing for her and whether she's outgrown a physical relationship with F and that it's time to move on?
There's a lot of interesting psychological development to be had that has been hinted at throughout the story. I fear however that you are right and that any pathway that doesn't end with F and D married with child is going to be left to die on the side of the road. And yes, it's not exactly what has been suggested if not downright promised over the last couple years.