It didn’t happen straight away. If she’d been Gallifreyan, her body would respond more obviously to the pheromones in his sweat, his saliva. His semen. She was human, though, related but not the same-- thanks to Rassilon’s tampering with the genotypes of the galaxy-- and it took a little time. Two weeks of sex. Of lovemaking, on his part, thankfully less frantic after the first time. Weeks of his body remaking itself into a machine for the production and protection of children.
He wasn’t sure what she felt about him. Oh, she loved him; he knew that. He’d known that for ages. She’d thrown herself into the timestream to save him. That was love, as she had reminded his idiotic self. But was it romantic love? Did she enjoy being in bed with him because it was him, and not just because of the physical pleasure? She’d fancied him in his previous face, his young face. Could she fancy this one? Because he’d long since given up trying to deny that he fancied her. She knew what Recognition was. She knew it meant he was head over heels.
He tried not to think about it too much, not that he succeeded. At least his body ensured that his fretting never interfered with the lovemaking. The sex. Whatever it was she thought they were doing, when his near-constant erection became something he could no longer ignore, when her scent drove him to distraction. Again.
He knew it when it happened. They were on the floor of the TARDIS library this time. He’d been showing her the books on the history of the Time War-- hundreds of contradictory accounts, all true-- when it got to him. Clara laughed at him and got them both undressed with practiced, calm hands. On the floor of the library, her head pillowed on his folded-up hoodie, her legs hooked over his shoulders. Perfect. He let himself push into her and sighed. Home, safe, doing what his body demanded.
“You’ve almost got muscles,” she said to him.
He looked down at himself. His hands, on her thighs. His arms. “I’ve got biceps,” he said, in some surprise. This body had always been so slender.
“And a bit of a six-pack.”
“Hmm,” he said. His gaze wandered down, to his now-flat stomach, and lower still, to where he was joined with her. He shifted his hips and watched himself slide out. He bit his lip. He was going to come well before she did if he did much more of that. Not that it mattered much; he’d be able to go again if she wanted it. He let himself watch. The pinnacle of Gallifreyan masculinity, or as close to it as a weedy scrawny man like he was could reach. Androgens in his blood, building him into the perfect protector. The perfect hunter. For a species that had no need to hunt and hadn’t for a million years. To hell with Rassilon and his genetic tinkering. To hell with his people. He was going to have a child with Clara, a half-human child, children, many children, and he was going to raise them right, raise them to care about the beings around them, care about people who were not Gallifreyan. He was going to sire children on Clara. He was going to come inside her again. He was going to fill her up, make her pregnant, he was going to–
“It’s this time,” he said, “it’s going to happen this time. Clara, oh Clara–”
And with that knowledge he came. Long seconds of orgasm, of his brain overloading with pleasure, of his body giving every last bit of himself to her.
When he came to, he was collapsed over her with his face pressed against her neck. “Sorry,” he said.
“You’re back. Yeah, that one was impressive to watch.”
“Did you, um?”
“No. It’s okay. I’ve kinda been having enough sex recently.”
He kissed her just under her ear, touched her with his tongue. Not quite yet, but she was fertile and it would happen soon, soon. He could scan her and confirm with technology what the brute biology of his body was telling him, but he knew.
She was smiling faintly at him. What she was thinking he didn’t know. He never knew what went on in that head. More than he had ever thought at first, that was sure.