Edmund knows Percy is an idiot. He takes advantage of it as often as he can. Like now, for instance.
Edmund hasn't been with a woman in over a month, with no prospect of getting one in the near future. Even the whores are refusing him now. He has needs. And his only options appear to be his manservants. Baldrick is out of the question, for obvious reasons.
So that only leaves Percy. The man's stupidity never fails to amaze Edmund. He has managed to convince Percy that doing this is a sure-fire way to curry favour with the Queen.
You'd think he'd learn, but apparently not. He falls for that one every time.