Hmmm, not really certain but I think the ML regressed. Their first marriage was probably a contract marriage and Ido thought Sejin was doing the marriage and seducing him to take advantage of him and his family's company to make Sejin's family's company more successful. So he probably disliked Sejin and did not treat him well. There must've been a lot of misunderstandings and Ido must've been an asshole to Sejin and only realized his mistakes after he lost Sejin.
I have a feeling that the Ido was also the cause of Sejin's family company to collapse after he regressed, so he would have an advantage when asking for Sejin's hand in marriage this time. He looks like he wanted to make it up to Sejin by taking him away from his abusive adoptive family and giving him the life he did not give Sejin before.
I also think Sejin was assaulted by Ido's older brother, hence his reaction when they met. Because Sejin's dreams are memories of a life he experienced before but not in the current timeline. Although, some of those dreams seem to be of him and Ido, but when he dreamt about a stranger raping him, I think it was Ido's brother.
Also, this is a sad thought but I think Sejin took his own life using Ido's gun or was probably shot by someone else using said gun and died that's why Ido threw out all the bullets and told Sejin not to come to the study room. Sejin probably died there.
Well, these are my own speculations but I'm curious if any of these will turn out to be true. Either way, I am preparing myself for the coming angst of the backstory.