When Hermione came up with her charm to stop the rain fogging Harry's glasses and thereby let them win the game, Oliver was overjoyed. So overjoyed, in fact, that he put a hand on each side of her face and kissed her soundly.
It was only when he heard her squeak of protest and saw her eyes, round with shock, that he seemed to remember she was just a young girl. He let her go, apologised profusely, and ran off.
They met again, some years later, long after Oliver had left Hogwarts and begun his successful career. By now, Hermione was no longer a child. And this time, Oliver did not run away.
If this was Roger, the sex wouldn't be so rough. If this was Roger, he wouldn't leave every assignation covered in scratches and bruises, wouldn't have to tell Madam Pomfrey they were Quidditch injuries, or listen to Percy tutting as he healed them.
If this was Roger, they'd be in a bed, in the Gryffindor or Ravenclaw dorm. They wouldn't be up against the changing room wall, or down in one of the spare dungeons.
But this isn't Roger. And every time Oliver slips, whenever he accidentally reveals his true desires, every time he cries another name, Marcus hits him.