Sunao-kun is an onahole and a toilet, so his duty to take loads from his Mistresses' dicks into his mouth or butt without spilling. If he spills it, he'll receive a severe punishment. But, even if he doesn't spill it, he'll receive a punishment regardless.

Sunao-kun's vigorous dick has no actual use, Mistresses tenderly treat it as a switch, which makes him burst out crying with a good scream if you torment it.