He is a prostitute. He might not be directly paid but he’s being pimped out my Michael to make money or possibly future favors for him. Prostitution is about an exchange in bodily transaction not necessarily money.
if you want to get into the specifics however he's actually 'whoring' himself because prostitution usually has the association with street corners and shady money deals, but whoring would be the umbrella of sex for exchange of something. I should also clarify that Michael is likely making money off of Sehyun through the gossip of his 'sex pet' to bring in bar clients or what have you.
Why does it seem so hard for yall to understand that sehyun ain't no prostitute! He ain't getting paid and it looks like that michel guy or whatever treats him as his property. Man this is sad