I doubt he has moved on. By threatening him, and showing his anger. It showed that it still affects him till now. If he truly has moved on, he wouldn't have done this.
If he had to have sex with the guy one last time to 'move on', then I don't think he'll be able to since he waited so long to do that. It's probably the jealousy from having married his sister that caused him to do this. So I don't think the seme would have moved on. He probably just wants 'revenge' to hurt the uke the way he believed he was hurt.
Me, the way I saw it, it's than they were going to cheat on the wife. "enter the forbidden world" But in a kinda rough way, not particularly beautiful: as, still love, but do no feelings sex, a painful decision. Because he said it was too late so he decide to make him suffer, even if he said than the uke have nothing to do for making it up. No idea if that would end well or bad tough…
That's the way I see it, but maybe in fact it's really the last time. (I prefer my vision, there a chance it goes well)
More of the 1st Story!! XD How can it be too late!!!