Prefects were a better sort of student. A better sort of person, really. Responsible, refined, dignified. Prefects were the backbone of the school. And they never broke rules.
Prefects did not sneak away during the middle of their hall patrols and backtrack to places they had already covered. They did not aim to convene with other prefects on patrol, because that would just be a waste of everyone's time. A prefect's time was precious.
So prefects did not take their girlfriend's hand and let her guide them through dark hallways. They did not slip through unlocked doors and stand in the library, which took on a whole different tone in the moonlight, becoming a sacred temple of books. Prefects knew nothing about temples.
They certainly did not slip into the Restricted Section and close the doors behind themselves. Prefects had nothing to hide behind closed doors, no views to block off and no noises to stifle. They had no use at all for hidden alcoves.
What they did have use for was uniform, a clean robe over a carefully pressed shirt and trousers. They had respect for the uniform; it was never tossed aside in a hurry, or bundled up and knelt on. Why would a prefect be kneeling? A prefect did not need to press their face against a warm skirt and breath out, feeling the heat hidden below the cloth and the twitches of the hands tangled in their hair. They were too dignified for messy hair. Hands had no place being tangled in it.
Hands had no place under skirts either. A good handshake was all the physical contact a prefect needed; a clap on the shoulder under extraordinary circumstances. They did not envelop their fingers in wetness or rub their thumbs against pinks bumps of flesh and nerves. Their ears were for listening to lectures or instructions, not listening to breathy moans and whispers of trivial nonsense like "come on, come on, please, oh please please, uh huh."
Tongues. In truth, were it not for the necessities of speech, prefects would not have any purpose for tongues. They were thick, ungainly, wet organs that were uncouth at best and obscene at worst. Having a tongue led to putting it all sorts of strange places. A person could end up pressing it into soft folds, exploring, licking and working in tandem with aforementioned fingers until the entire landscape before a person's eyes began to twitch and shake, accompanied by even more moans and gasps and a thigh lifted up and balanced across a person's shoulder. A person could be driven to distraction with a tongue. Not a prefect, though.
A prefect would never be caught lying on the floor of the library on a spread-out cloak, holding a member of the opposite gender in their arms. The smell of sweat and sex and parchment and ink would take forever to get out of one's clothing, and even if it was appropriately purged, the memory of the scent would do far more lasting damage, popping into the mind at the most inopportune time for weeks to come. Prefects could not afford to breathe deeply and absorb it all, to relish in the feeling.
Prefects could not do any of these things.
Fortunately for Percy, Head Boys could.