"I'm not an idiot, Black. I understand the mechanics. We can use your robe, it's bigger and always covered in mess anyhow." Severus broke the contact with Sirius, pushing him back light as he wandered towards the other boys clothes. Sirius could look at his arse all he wanted when be bent over. He fanned out the robe and turned back towards Sirius.

"I believe kneeling will be easier."

Yes. He expects Sirius to bottom.