Giles had never thought of Buffy sexually until she presented herself to him as such. Only then did his mind allow him to acknowledge how attractive she was, allow him to be attracted to her. It felt wrong because of her age, because he was her teacher, because he was still mourning Jenny. He resented it because he wanted to mourn Jenny, and to keep those feelings sacrosanct. Yet at the same time it was a strange relief to suddenly feel attracted to someone else. There was also a part of him that wanted Buffy because he wasn't supposed to, a part of him that was tired of all the grief and responsibility in his life and wanted to abandon it all and embrace her because she offered rebellion and pleasure. Giles hated himself for it.
The next day he tried to convince himself that he was repulsed by the idea of sleeping with Buffy, but then he would look at her, and it was impossible for him to believe that making love to her could be anything other than beautiful.
Buffy noticed right away that he was acting differently around her. He was awkward; he stuttered more and fumbled with his books. He was trying not to look at her, but every now and then their eyes would meet and he would become even more flustered. She liked doing this to him. She didn't want to admit it to herself, but it gave her a sense of power, and with other parts of her life in such a mess that power made her feel good. They were going to do this, she would make it happen. She told herself they were going to do it because she wanted to do it for him, not in spite of him.
Whenever they met throughout the day Buffy stood too close to Giles and looked at him winsomely. His resolve waned a little more every time he looked at her. He began to feel helpless to do anything but follow what was apparently her plan.
At the end of their training session Buffy perched on the edge of the study table, and leaned back slightly, very aware of how her body looked in her tank top and miniskirt. As he put the weapons away, Giles couldn't help but imagine taking her in his arms and kissing her, running his hands over her body, making love to her…
"So, what now?" she asked.
His voice felt caught in his throat. "Shall I drive you home?" he choked out.
She looked at him blankly. That wasn't what she wanted. "You mean to your place?" she said quietly.
Giles thought he should reply, "No," that he should let her get home on her own, that he should say something that would put an end to this right now, but his mind refused to work and he couldn't think of what to say.
Instead he simply left the library and she followed him.