Sanji makes a pleased sound from the back of his throat, making it vibrate in the process. Given that Luffy's cock is hallways down, he probably felt it too. The moan his captain lets out goes straight to Sanji's own hardness, heat pooling on his lower belly.

Careful not to displace the hand on his head, Snaji pulls back a little. He slips the cock out of his mouth so he can roll his tongue all the way around the head at the same time he's stroking Luffy up and down. He wants to draw out more sound out of him, and make Luffy think about nothing but him, weak in the knees with bursts of hot pleasure. Maybe it's selfish to want someone this much, but Sanji hardly ever had something or someone to call his own, and he's not willing to give up Luffy easily.