Charlie's strokes were nearly perfect. He hit Severus' prostate with a frequency that made Severus nearly insane with pleasure.
Severus caterwauled when he felt his balls drawing up tight to his body. He came with a scream that had to have been heard outside of the building. His body bucked and writhed, and clawed the sheets as he ejaculated on himself.
He wanted Charlie to never leave if their sex was going to be this good.