He bit his lip rather hard as she settled so neatly exactly where his body really wanted her to be. "Not, not with me," he said shakely. "It's in my trunk." As there was no way in hell that he was going to be able to walk all the way up to Ravenclaw Tower to get it, he knew that the charms were their best option (since not following through on this opportunity was not even worth concidering).
Terry had been trying to gather his wits enough to offer to cast said charms when her skirt came off and his hands slid down to automatically settle on her bare hips. Susan wearing nothing but her knickers and him in just his trousers and, and... was it getting hot in the alcove or was it just him?
"Please stay still for a moment," he muttered, closing his eyes and running formulas in his mind until his nerves and body had settled enough that he felt secure enough to sit up with her still in his lap so he could reach for his wand where it lay tangled in the sleeve of his robes.
Then with very precise gestures and words, he cast the most effective of the contraceptive charms on both Susan and himself. Setting his wand aside once more, he smiled at her crookedly. "There," he whispered, "safe as houses." Terry drew her closer once more so he could kiss her longingly.