He was just teasing, he didn't really know what it'd lead to.
"Shun, what if I was a vampire?" Yuki wondered aloud as they sat on Shun's couch, watching some old Halloween thriller movie. After all, it was that time of year. "Would you run away?"
"Of course not!" Shun replied cheerfully, smiling sweetly to Yuki. "I'd be your friend no matter what, Yuki. That's what friends are for, right?"
"Even if I was a blood-sucking monster?" Yuki murmured, eyes trailing to Shun curiously. The redhead nodded and smiled, his cheeks that rosy-pink color that glowed happiness.
"Yeah! You'd still be Yuki, right?"
"Of course." He looked away before smirking a little to himself. "And what would you do if I bit you?"
Shun was caught off guard by the suggestion. "Bit me?" He cocked his head to the side and pressed a finger to his lips in thought. "Mm… I'd… it'd be okay." He turned to Yuki, still smiling. "As long as it helps you. Live, or stop hunger." Yuki raised an eyebrow, then suddenly leaped onto Shun, pinning him to the bed in a playful scuffle. Shun blushed and looked up to Yuki.
"Yuki, what are y— ah!" He squeaked as his breath was taken away; teeth sunk into his neck, and Yuki was sucking at his soft skin before pulling pack, licking his lips playfully. Shun's face flushed red, and he stammered, looking to Yuki, not sure how to react or what to say.
"Like that?" Yuki smirked, suddenly feeling the need to take control. "What if I bit you like that all the time?" Yuki leaned down and bit him again, in a different spot, this time gradually applying pressure on his delicate skin.
"Y-Yuki!" Shun cried out, the pain strangely turning him on. "Th-that hurts!" As much as it hurt, Shun had an odd desire for more. Though, before he could beg, Yuki pushed himself off, sitting up and looking back to the TV, waiting to see what would happen. Shun pouted, slumping a bit before sitting up and gripping Yuki's shoulders.
Yuki turned in slight surprise. "Shun, what are you—!" he gasped as Shun bit down on his neck, not as hard as Yuki, but a smaller, tighter grip, and he pulled back, looking at Yuki's eyes before smiling sweetly and sitting down next to Yuki.
Yuki rubbed the mark on his neck, blushing and grunting as they continued the movie. "Tease," he scoffed, taking a handful of popcorn and eating it.
"I could be a vampire, too, you know," Shun suggested, a small giggle laced in his voice, "Even if I don't look like it.~" Yuki stared him in awe before they both started to laugh, their shoulders shaking before they calmed down to continue their old Halloween movie.