Because this is a fictional manhwa, girl. Authors can make the most unlikely pairs kiss each other. It doesn't always need to make sense. If fictional ships were made only by keeping real life realism, romance would be boring as hell. The rape was the only issue in this manhwa, the rest was good nevertheless.
Firstly, this manhwa isn't at all romantic. There are plenty of interesting, not-boring-as-hell romances in violent settings or otherwise, that are non-toxic and completely wholesome. I just like my couples to not only love and be devoted to each other, but to also be good, kind, understanding and considerate towards each other. Consent is the minimum in romance. However, if you find a manipulative, inconsiderate and controlling partner who rapes and constantly sexually harass his weaker and timid partner romantic, then to each their own I guess.
How can a sweet and kind guy who strived to live a normal life away from his mafia family easily fell for a violent, immoral, selfish, manipulative, controlling gangster who constantly sexually harassed him, ignores his words, dismissed his feelings and just treat him so inconsiderably?