Cloud shifts suddenly, but Sephiroth reflexively accommodates for it. More to the point - and much more relevant to Sephiroth - is that the shift and the sounds he makes certainly don't seem to be ones of disapproval. And if Cloud doesn't want him to stop...Sephiroth doesn't feel he has much reason to.

He's never done this before himself on another man, but Genesis has done it for him and Sephiroth's filed away at least some things from the experience. Between that and being exceedingly careful - Cloud has made it clear he's uninterested in pain, and Sephiroth is focused on giving him only pleasure here; it feels important to him in ways he can't even define with the rain washing away further context - he's making a pretty good start. And, as Cloud watches, Sephiroth lowers his head to take Cloud fully into his mouth, sucking experimentally.