Responses
I'm pretty sure what he meant is that Kouta was making such an erotic face and showing that he felt so good that Motoharu was almost "jealous of his own dick" as in if that's how good Kouta was feeling then he was almost jealous enough to want to switch places. (≧∀≦)
I believe he was referring to the face Kouta was making while looking at his dick. He was jealous of the way Kouta was looking at it.
Right!
Like, 'which do you love more? Me, or my penis?'
I'm not sure that makes any sense, but it's super memorable as dialog goes.