"I don't know that I could hold back any longer if I wanted to," Sephiroth replies, eyes dark as he watches Zechs. His pupils are already significantly dilated just to cope with the low light, but that's probably not the only reason at this point. "And I don't want to."

Fortunately, Zechs has already made it clear that he won't let Sephiroth hurt him more than Sephiroth is comfortable with, and will let him know if he crosses any lines. Knowing that is a relief. Of course he has to do his best to control himself anyway - Sephiroth would rather not only find the line after he's already crossed it - but it's nice to be assured that he'll be reined in if he starts losing himself too much. Paradoxically, that makes it easier for him to let go.

And it really is letting go. Sephiroth doesn't even wait long enough to offer acknowledgment to what is clearly Zechs' go-ahead; even if anything else needed to be said, Sephiroth isn't interested in wasting the time to say it. He simply pulls his hand back from Zechs' cock to brace his own, lines himself up, and then pushes into Zechs in one long, smooth stroke.