This story is placed towards the end of Chapter 14 in City of Glass. Clary is in her room at Amatis's house in Alicante, and it's the night before Valentine returns with his proposal for the city. Jace climbs up to Clary's room, and declares his love for her. One thing leads to another.
"There is no pretending," Jace said with absolute clarity. "I love you, and I will love you until I die, and if there's a life after that, I'll love you then."
She caught her breath. He had said it – the words there was no going back from. She struggled for a reply, but none came.
"And I know you think I just want to be with you to – to show myself what a monster I am," he said. "And maybe I am a monster. I don't know that answer to that. But what I do know is that even if there's demon blood inside me, there is human blood inside me as well. And I couldn't love you like I do if I wasn't at least a little bit human. Because demons want. But they don't love. And I – "
He stood up then, with a sort of violent suddenness, and crossed the room to the window. He looked lost.
"Jace?" Clary said, alarmed. "What's wrong?"
"I shouldn't have told you like that," he said, not looking at her. "I'm sorry, that was probably a lot to take in. You looked so… shocked." The tension underlying his voice was a live wire.
"I was," she said. "I've spent the past few days wondering if you hated me. And then I saw you tonight and I was pretty sure you did."
"Hated you?" he echoed, looking bewildered. He reached out then and touched her face, lightly, just the tips of his fingers against her skin. "I told you I couldn't sleep. Tomorrow by midnight we'll either be at war or under Valentine's rule. This could be the last night of our lives, certainly the last even barely ordinary one. The last night we go to sleep and get up just as we always have. All I could think was that I wanted to spend it with you."
Her heart skipped a beat. "Jace – "
"I don't mean it like that," he said. "I won't touch you, not if you don't want me to. I know it's wrong – God, it's all kinds of wrong – but I just want to lie down with you and wake up with you, just once, just once ever in my life." There was desperation in his voice. "It's just this one night. In the grand scheme of things, how much can one night matter?"
Because think how we'll feel in the morning. Think how much worse it will be pretending that we don't mean anything to each other in front of everyone else after we've spent the night together, even if all we do is sleep. It's like having just a little bit of a drug – it only makes you want more.
But she would want more no matter what, she realized. She wanted to do more than just sleep. There was nothing she had ever wanted in her life more than this night with Jace.
"Close the curtains, then, before you come to bed," she said. "I can't sleep with this much light in the room."
The look that washed over his face was pure incredulity. He really hadn't expected her to say yes, Calry realized in surprise, and a moment later he had caught her and hugged her to him, his face buried in her still-messy-from-sleep-hair. "Clary…"
"Come to bed," she said softly. "It's late." She drew away from him and drew the covers up to her waist. Somehow, looking at him like this, she could almost imagine that things were different, that it was many years from now and they'd been together so long that they'd done this a hundred times, that every night belonged to them, not just this one. She watched him as he reached to jerk the curtains shut and then unzipped his white jacket and hung it over the back of a chair. He was wearing a pale gray T-shirt underneath, and the Marks that twined his bare arms shone darkly as he unbuckled his weapons belt and laid it on the floor. Clary could see the outlines of his lean muscles under the thin shirt. He unlaced his boots and stepped out of them as he came towards the bed, and he stretched out very carefully beside Clary. Lying on his back, he turned his head to look at her.
"Good night, Clary," he said.
His hands lay flat on either side of him, his arms at his sides. He seemed to be barely breathing; she wasn't sure she was breathing herself. She slid her own hand across the bed and laced her fingers with his. "I don't want to sleep just yet," she whispered softly.
His face was unreadable. "What do you want to do then ?"
"I want to – " she hesitated. "I want to talk."
"About what?" He had opened his eyes now, and they were gazing at her.
"I don't know… you think of something first." She hadn't really thought about what they would talk about, she just didn't want to simply lie there in silence as they waited to fall asleep.
"Did you really kiss Sebastian?" Jace asked in a firm tone. There was a hard look on his face.
Clary was caught by total surprise. Her eyes widened, and then she looked down to avoid meeting his eyes. "Yes… but I didn't want to – I mean, I wasn't – I just – " she stammered helplessly.
"Was he good?" Jace interrupted her stuttering. The expression on his face hadn't changed. "Was he better than me?"
Clary was surprised yet again. "I –" she started, but didn't get to finish.
Jace rolled over on the bed and ended up on top of her, his knees on either side of her waist, and his hands pressing down on the mattress by both sides of her head. His face hovered above hers, and he leaned down – very, very slowly – and gently pressed his lips against hers. There wasn't much pressure, but the kiss lasted a while.
"Better than that?" Jace asked again. His tried to keep his expression the same, but Clary could practically hear his heart beating out of his chest, and they were both a little out of breath.
Clary was speechless. She quickly shook her head no. Then, without even realizing what she was doing, he reached out both of her arms and placed them on Jace's back, bringing him down towards her until he was lying on top of her. His breath was hot against her ear. She turned so that she was facing him. This time, she was the one who kissed him. She felt him tense and then relax. Taking this as a sign, she deepened the kiss, and she felt a sudden lust stir inside her. She knew it was wrong, that all this was wrong, but at that moment, she wasn't even thinking about it.
Jace ran his hands through her long red curls, and slipped his other hand down her side so that it rested at her waist. Clary felt shivers up her spine, her body reacting to Jace's touch. The kiss grew hungrier, and Clary opened her mouth to let out a small moan. Jace took this opportunity to let his tongue slip inside her mouth, and Clary gasped in surprise. They explored each other's mouths, and their tongues danced together.
It seemed like it had been forever when the kiss broke off, and they were both breathing heavily. Jace leaned in again, and this time, he planted small kisses along her jawbone all the way up to her ear. A sound escaped from Clary's lips, and he quickly caught her lips in a chaste kiss, trapping the sound. She ran her hands through his golden locks while he explored her body. His hands traveled lower until they reached the bottom of her thin camisole, where they started to move up again, under the cami.
"Jace…" Clary whispered.
Jace immediately pulled away. Clary could see from his face that he was just as nervous as she was. His cheeks burned a bright pink, and she was sure hers were the same. "Do you want me to stop?" He asked softly, but Clary couldn't help but notice that his face fell.
In reply, Clary took his hands, and guided them back to under her shirt. She sat up a little, and with her hands still on his, she helped him to take off her cami. It was then that she suddenly felt very exposed, and tried her best to cover herself with her arms.
"Don't," Jace said, staring at her. He slowly grasped her forearms and moved them out of the way. "You're beautiful."
Clary was shocked, flushed a deep shade of red, and looked down in embarrassment. When she looked back up, she found that Jace had busied himself with the task of removing his own shirt, and when he did, she was mesmerized. She traced her fingertips lightly against the dozens of small white scars that covered his chest. She bent to gently kiss every one of them, then looked up at Jace's face.
Jace gave her a crooked smile. "So now that we're both half naked, what do you want to do?"