Ah - now he's hearing Cloud, and that stirs something hot and possessive in Sephiroth. The knowledge that Cloud is making those sounds because of him, for him...he wants more of that, pursues it by redoubling his efforts, focusing on the things that make Cloud react the most positively.

The jerk of Cloud's hips up into his mouth jolts him, though; he's not at all experienced in doing this, isn't prepared for it, and it comes close to choking him in his unprepared state. But he quickly fixes that by...putting his hands on Cloud's hips and holding them down, assuring Cloud can't move without Sephiroth's controlling it.