Gyuwon has never done any sort of BDSM, and he's practically a virgin in that he's never had sex with anyone (this is shown in Ch 5-6 of the webtoon, the next few ones). He's inexperienced, and all he knows of sex and BDSM is from media and porn, so in fact, after each of their plays, Hwayoung always does aftercare. Gyuwon doesn't know that aftercare is a thing lol.
Actually, you'll notice later on that Hwayoung is nice and friendly outside of their plays, but coldhearted during. That's not to say that he goes too far, it's just that in the novel, they make it out that Hwayoung is really, really good at reading Gyuwon. But later on, they do talk about what plays/kinks they (mainly Gyuwon) like and dislike, so their relationship is pretty healthy as far as communication and being respectful of the other.