So considering only 4 chapters have been translated, I'm really curious on why there's so many 1s lol. For the 4 chapters that exist, it's really not bad. The only reason I can think of - and I hope it's not because of it - is that the main female lead sleeps around (not cheating, just one-night stands). And if the ratings are that low because of such a thing, I'd be extremely disappointed. It's basically the college-age, adult version of a troubled tsundere. She clearly has intimacy issues, and so drowns it in sex, which is a ridiculously common thing in real life.
If people are giving it 1's because of such a incel, white knight reason, I'm saddened at how immature people can be.
As for the series itself, the MC is a nice guy who gets swept up in her for a one-night-stand, and then later it turns out his dad is remarrying and she's his new step-sister. While that's not a common plot (it's actually super rare), it has happened very recently very publicly, so it's understandable we see another 'copy'.
Anyway, the romance seems nice, with him holding back and her actually getting some emotions over the act. I see the series is complete at 14 chapters though, so I'll be interested to see how it turns out (for all I know, it might crash and burn and deserve really low ratings, but for now, the 1's are completely undeserved).