Prompt: Twilight gave Spike a stern look. "Did you really think I wouldn't notice? This is the third time I've woken up with semen inside me!" 
        Spike looked around nervously. "I-It wasn't me, I swear!" Twilight huffed at him. 
        "I know what dragon cum smells like! You... I know baby dragons have certain urges, but that doesn't excuse... using me, like I'm some kind of sex toy!"
Memory: [Twilight Sparkle is a purple unicorn.]
        [Spike is a baby dragon. He lives with Twilight Sparkle.]

Twilight gave Spike a stern look. "Did you really think I wouldn't notice? This is the third time I've woken up with semen inside me!" 
Spike looked around nervously. "I-It wasn't me, I swear!" Twilight huffed at him. 
"I know what dragon cum smells like! You... I know baby dragons have certain urges, but that doesn't excuse... using me, like I'm some kind of sex toy!" She shivered as she remembered how he had used her to get off and then went back to sleep. 
"You're right," said Spike apologetically. "That was wrong of me." He took a deep breath and continued in a more serious tone. "But I can explain..."
"How?" asked Twilight skeptically.
He sighed. "Well, first thing's first: I didn't come here for your body."
Twilight frowned. "Then why are you here?"
Spike shook his head. "No, no, listen. The truth is... well... it started out just being about my own pleasure, which is true enough. But now there's something else going on."
"What do you mean?" asked Twilight.
Spike bit his lip. "Okay, so, this whole 'being turned into a sex doll' idea has been floating around lately. And it got me thinking - maybe I could be useful for something other than getting off or making love."
"Useful?"
"Yes. Maybe if I were an android, or even a robot, I'd be able to help the Mane Six. They always need things done for them; they never seem to have any free time. Well, not since Applejack left town anyway."
"And you want to become one of these helpful robots?"
"Not exactly, although I am interested in the technology involved. No, I don't want to turn myself into a machine. What I meant by that was becoming their personal assistant, someone who would take care of all the chores they couldn't handle themselves. Someone like, oh, say, you."
Twilight stared at him blankly. "Me? Why me?"
"Because you're smart. You're capable. You've shown yourself to be incredibly resourceful when dealing with difficult problems. Plus, we both know you're good friends with Princess Celestia."
She thought about it. It sounded pretty good. "Yeah, I guess it does make sense. So, how would I go about doing that?"
"First step would be convincing Luna to let us test our prototype on you."
"Luna?"
Spike nodded. "We can use her as a guinea pig. If everything works out, we'll talk to her again later. In fact, I should probably go see her right away."
Twilight gave him a dubious look. "Why? Is she okay?"
Spike shrugged. "I think she'll live. I'm sure she's fine. She's just unconscious right now, remember?" 
The two ponies walked through Canterlot towards Twilight's house. Spike was still feeling nervous, despite his best efforts. He hadn't expected Twilight to agree so easily. He hoped he wasn't putting her in danger. 
They reached Twilight's front door and Spike knocked. When Twilight answered, he saw that Luna was indeed awake and sitting up in bed. 
"Hi, Spike," said Twilight. "Is everything alright?"
Spike smiled awkwardly at her. "Uh, yeah. Everything's great." He turned to Luna. "Princess Luna! Are you... are you okay?"
"It's a little strange, but I'm alive." She rubbed her forehead. "It's a lot less painful than I expected. I'm glad I didn't die."
Spike looked relieved. "Good! That's... um... very reassuring!"
He glanced over his shoulder at Twilight Sparkle. "So... uh... what happened?"
Twilight explained how the attack had come from behind, how she had been forced to fight, how she had managed to escape.
Spike listened intently.