You get an idea. Slipping three fingers into him, you crook your fingers and coat your hand in cum before slowly pulling it out, making sure to drag your palm and finger against his clit. You bring it up to your face and pretend to study the fluorescent liquid.
“Pet-?”
“Thank you, master.” You moan before messily lapping up the mess on your hand. You don't mind it, obviously, and besides, he tastes like artificial sour cherries.
That got him going.
You hear the fans speed up, now sounding like an old laptop trying to run more than two programs. He’s blushing furiously and his eye is glitching out, displaying distorted hearts and occasionally, his usual pupil. He attempts to say something, but only stuttered moans and broken syllables of your name come out. You smirk and finish cleaning your hand off before returning to dip your fingers back inside of him, pretending that never happened despite the large amount of pink goop on your chin and lips.
Mettaton looks mildly offended at how well you pulled that stunt off. Mildly offended and extremely turned on. He quickly regains his composure and slowly runs his hands down your sides, sucking bruises into your skin and nipping gently around your jawbone. When he pulls back you're fucking terrified of the downright evil smirk plastered on his face.