For Haneul (the FL) it was just curiosity at first and for Mincheol (the ML) he had always somewhat liked her. The problem is that he knew, for her part, exploring sex is just out of curiosity so he doesn't want to hold any expectations and ruin their relationship. On the other hand, Haneul starts to see him as a romantic interest but is seeking the right time to confess ╮(￣▽￣)╭
It's because they've been friends for 20 years and they don't want that to end just because of an impulse decision. And the other reason is still unknown the author haven't revealed yet, but the clue is Mincheol might have done something or accidentally to the FL when they were younger, so the guilt is there and it's holding him back.
And about the sex they're both pretty open about it, since the FL is curious about sex and how it feels that's why they both agreed to do it, with the condition of not avoiding each other after. But she realizes she likes him and was trying to get closer to him and be in a relationship. But Mincheol is afraid of what might happen with their relationship as friends and maybe soon a couple.
they’re so casual with letting their bodies touch that it goes further and have sex, why are they not dating? What’s going on? I’m a scroller so I just quickly scan and that’s it but did the story ever say anything about why they’re not dating but will go further and kiss and sex? Like come on bruh if you like each other then don’t waste time and date.