A/N: So I evidently don't know the meaning of a one shot. This just sort of got written, so I'm adding it on as an additional chapter.
Yami sat for a moment, stunned. In spite of the fact that he had taken a while to get into the rhythm of things, despite the jarring sensation at the end when he came back into himself and realized that it was indeed not Seto, it had been enjoyable. And for that he felt guilty. How truly did he care about Seto if he could use a substitute like this and still get off? If he would use the other's name without his permission, with someone else that he didn't love, just for pleasure?
The shower turned off and he heard footsteps head into Yugi's room and the door close. He examined his chest and found that he was well-cleaned from the wipe down he had received earlier. He figured a shower wasn't necessary and slowly began gathering his clothes.
Yami noticed something was wrong when Yugi didn't come out of his room after twenty minutes. He wondered if it was his worst fear about the interaction being realized - that Yugi would get hurt. As his mind leapt to the conclusion, additional guilt ate at him: How could I have done this to aibou? How could I have let him convince me to do this? But the truth of the matter was that Yugi had said he'd be fine, it was his idea, and Yami had chosen to take him at his word. Somewhere underneath the critical thoughts, Yami was aware of this, and knew that he was not at fault for what Yugi was likely experiencing, but it didn't help make him feel less guilty. How many people will I hurt with this action? Yami thought.
His guilt only increased thinking back on the act once more. It had felt good. He hadn't had a release like that before, and calling Seto's name had been quite cathartic. Despite the awkward ending to the experience, he could not deny the powerful feelings that had come along with it. It was as if he had been sideswiped. He was left confused: he still liked Seto, there was no question about it, but he craved experiencing the same pleasure again. And that meant Yugi. Still, if the other was hurt, there was no way that he would do anything to hurt the other intentionally.
Yami sat with the guilt, simultaneously wishing that it hadn't happened for Yugi's sake and being thankful that it had for his own. Simultaneously worrying for Yugi and wanting what he thought could only hurt him more; feeling both as an older brother and something else entirely. It was exhausting, it was difficult to understand, bothersome. So he waited. Waited for his head to clear, waited for Yugi to come out of his room, waited for some cue as to what his next action should be.
Yugi cried softly in the shower. How did Joey stand it? Hearing Yami's name on my lips, filled with the same wanton lust that I had heard in Yami's. Even worse, those first few times - actions he recognized taking when he was in Yami's position himself - the calls meant to convince the imagination. The calls when Yugi must have been too small, or hesitant, or different from Yami's initial imagination. When Yugi couldn't convincingly be Seto, because he wasn't. He could never fill that gap.
He realized he was selfish. Selfish for wanting what wasn't his, and selfish for thinking he could ever be right for Yami. Guilt raked Yugi as he thought of how Yami must be feeling: unfulfilled, longing, even horrified. No replacement was possible - Joey hadn't been able to replace Yami anymore than Yugi could replace Seto. And that look of surprise on Yami's face after he came back into himself and realized what was going on. How could he have been so stupid? Yugi couldn't stand it. He had to have hurt Yami, and he wasn't sure he could deal with it.
And yet, he couldn't deny that it was nice to be with him, as selfish as it was, to have seen Yami's writhing figure below him - glorious, pleasured expressions gracing the other's face. For a moment everything had been perfect - no mention of Seto due to incapacity to speak, just grunts and groans and pleasure on both sides. Yugi was sucked into the memory and felt himself growing hard thinking about it. But the more he remembered Yami's reactions, the guiltier he became.