As a Matter of Fact
Harry has learned that Snape does not like foreplay. Not one bit. It is a waste of time. An utter waste. That is what he would say if Harry bothered to ask him. Harry has not because he finds he is accustomed to fast nights or evenings or lunch breaks strewn over a desk or pressed against a door. Snape's ways have been driven into him. Snape is a forceful man.
Harry and Snape schedule sexual intercourse for every Wednesday and Friday. Sometimes they cannot help meeting in between as well. They are not sensual or romantic affairs with fine drinks, rose petals, or warm oils. Not with silk sheets or satin ones or any other pleasant material. Cotton. Harry's slept on cotton all his life. He is accustomed to that, too. Not that he's ever slept in Snape's bed. He's never seen the inside of Snape's chambers. They tend to lay themselves out on the clothes they shed.
Snape does not touch Harry often, except on the hips for leverage and to spread apart his buttocks. Harry wonders why he can stick his hands into jars of living, wriggling specimens, but finds Harry's skin so repulsive. Harry bathes. He's even sniffed himself and he thinks he smells fine. He wonders if Snape would wear latex gloves and a hairnet during intercourse if there were time.
Snape plugs him up in a firm movement and grunts and humps until he has neatly deposited his ejaculate into Harry. There is nothing left behind when they are finished, not even lubricant. Snape wipes it off his penis and offers the handkerchief to Harry. Polite man. Snape never demands his handkerchief back once Harry has used it.