The novel isn't mature so the sex scenes would be skipped so if you're curious about their sex I can give people Chima (the author) posttype where she posted their 18+ short story, for those who didn't know to access i would write what happens
In summary their dynamic is so predictable you can imagine it inside your head bc of their trope with forceful seme x tsundere uke lmao.
Heejae purposely making noona horny despite noona being tired. He asked to do it everyday but noona said only once a week, but then it's not heejae if he's not doing some manipulating game lol, he purposely masturbate on top of noona's sleeping body (he's not really sleeping) and Noona got aroused bc of his grunting voice. Heejae loves attacking noona's nipple. And his dicc is big so noona said the first time was really difficult for him bc heejae was too stubborn (idk what it means maybe in thrusting he's stubborn lol) but well, our noona is a tsundere despite complaining he still said inwardly "it's not that i don't like it he's a pervert!" But still moan and do boombayah anyway