Um," you glanced up at your bound wrists, "my hands are tied on this one."
"...OKAY, YOU ARE DEFINITELY BEING PUNISHED FOR THAT ONE." Despite the threat, he seemed to enjoy your joke. "HERE, SAY 'AHH'" You opened your mouth.
"Ahh-?" He shoved the heart in your mouth, much to your surprise.
"HOLD ONTO THIS FOR A MINUTE." Oh man. It was tooth-achingly sweet, with mixtures of bubblegum, fruit punch, and a flavor you couldn't identify. Ectoplasm? And... copper pennies. Whatever it was, it made you want to swallow it whole. You sucked gently on it, little drips of pink goo spilling down your throat and dribbling down your chin. Mettaton seemed to shiver, fiddling at your buckle. It took him a few seconds to get it undone, and then another second to undo the button and pull down the zipper. He paused for a moment, breathing shakily. "D-DARLING, I MUST ASK THAT YOU REFRAIN FR-FROM SUCKING ON THAT UNTIL I AM DONE."
Oh? You rubbed your tongue all over what you could. He let out a breathy moan. "PLEASE-" You sucked on it harder, more of it falling into your mouth, "-JUST-" he unsteadily angled himself so he could lower your pants, "-BE GENTLE-!" you swallowed more of the goo, "-OH, OH GOD THAT IS-" he swore, finally nabbing the heart out of your mouth. It glowed a dull light, throbbing and buzzing a bit. He panted heavily. You stifled a giggle. The taste of your blood began mixing with the sweetness, turning you dizzy. He gave a sinister glare, flickering his expression between that and an equally malicious smile. "YOU'RE IN FOR IT NOW."
Your pants and undergarments had only been taken halfway off, limiting the mobility of your legs. Mettaton knew what he was doing. He traced a line of warm pink goo down your torso, stopping right above your sensitive area. You rattled, feeling the heat the heart gave off. Then he decided, no, not yet, and avoided your genitals and pressed the heart to the inside of your thigh.
"Ah, shit," you sighed. The vibrations of the heart intensified, sending short waves of pleasure through you. You couldn't hold back any longer, letting out soft 'oh's. Mettaton's head lowered, bending over take you in. You stopped looking at this point. All you know is you could feel his teeth scraping along your other thigh, both the heart and his mouth creeping their way towards your privates. The heart reached there first, the vibrations sending you into twisting convulsions of pleasure. Your moans grew in volume and frequency. You arched your hips, stimulating a moan from Mettaton as well.