It's wrong even if Jade initiates and Heine knows it, but all the same he can't seem to resist much more. There's clearly nothing there in terms of emotions. Jade knows what he wants.

Heine lets him open the front of the robe this time, just watching his hands. Despite the clothes underneath it he still feels oddly exposed.

"...then do it already." So they can go back to being professor and student, so that Heine can force the inevitable memories of this into a box that he'll try not to open ever again. There's a slightly heavier edge to his breathing than is usual, and he doesn't know where to look, but he doesn't move away again.