As he returned her kiss -- one hand tangled in her hair, the other digging its fingertips in her back -- Severus felt his heart soar, even above the Astronomy tower. His Mary loved him too, and all was right with the world. For several long seconds whilst they kissed, there was nothing in that world apart from the two of them, and their lips and tongues, and their bodies pressed together, clinging to one another for comfort and reassurance and yes, love.
After a few more seconds, however, Severus became aware of the sounds of people moving past them in the corridor, and a few whispers, as well as scandalized or disgusted noises. At that point, he thought they really ought to stop. But then again, snogging her so obviously and brazenly would also show yet again how wrong Rita Skeeter and her rash conclusions were. And so he decided only to stop when Mary pulled away, deciding that she'd had enough.