Harry was joking when he thought that. Draco was taking so long about making love to him that he believed he'd seize any excuse to take still longer.

Draco hasn't been allowed into that room yet. Harry wasn't sure what he would do, either, and since then, there's been more than enough to occupy them. Draco might have thought about the Dark magic he felt, but he hasn't discovered the right time to ask Harry about it.