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I saw this under a comment so here's your answer:
summary: there's no cheating
long story and more spoilers:
Yeonseo has had a crush on Soohan since childhood but her love was one-sided. Soohan was born in a bad family environment, he was abandoned-not taken care of by his own parents, when Soohan was still a school student, he moved to Yeonseo's house and was raised by Yeonseo's parents. Even though Soohan is smart, there are still many envious people who don't like him and often underestimate him, Soohan was even called cheap when he manifested as an Omega. When Soohan was 20 years old and studying abroad, one day he visited a large family event at the Yeonseo family villa. There were 3 dog shit bastards from the Yeonseo family who tried to rape Soohan while he was sleeping, because our uke was really badass and had a good body, those 3 people beat him up as self-defense. Unfortunately Soohan couldn't see the perpetrator's face because he was sleeping with a sleeping mask covering his eyes, so he assumed the perpetrator was Yeonseo and another alpha from the family. Since then, Soohan has really hated Yeonseo. Even though Soohan hates Yeonseo, he loves his parents because they have supported and helped Soohan since he was at school. Yeonseo's parents also love Soohan to the point of encouraging Soohan to marry their son. Soohan certainly doesn't feel comfortable refusing his request to marry Yeonseo but Soohan pretends he doesn't exist, Yeonseo's heart is really broken into pieces and that's why he's really careful about treating Soohan even though Soohan forgets his memory.
Still in flashback some time later Soohan went to Yeonseo's family villa again to investigate the truth about her rape incident, but there was an accident and the perpetrator was killed.
From that accident, Soohan lost his memory. As time went on, their relationship seemed to start to improve (still in Soohan's memory loss state) the two of them started having sex quite often until finally Soohan became pregnant. Slowly his memory came back because during morning sickness Soohan wanted to eat the food that Yeonseo prepared and the story had a happy ending (from user Calyna)