I don't think it's lack of interest, it's the realization they made an absolute incompatibile pairing, but still stick to the delusion of the regular "happy ending".
And since any change on MC that makes him deep dive being more and more mazochistic in order of that "happy ending" would justmean his traumas are ruining him, the only solution remains that ML sadistic tenencies magically disappears, which is... well, would be equally terrible from the other side of the spectrum.
They should have just break up, that would be the only logical and desirable outcme of this. to learn where their borders and red lines are, and realize they're not compatible, but with this new knowledge, they can find their right persons in their next relationship. TRHAT would be a real happy ending.
Sad, the author didn't dare to risk the BL rule of endigs. :really sad. :(