
Spoiler....
So you know marking bonds both their souls and the most important thing is to have consent on both sides.
But Ido forcefully bonded Sejin. (He actually flipped Cuz Sejin asked for a divorce. That idiot)
But Sejin was unwilling.
After mutual bonding, the partners can feel each other's emotions since they become connected but in one sided bonding one of the partners, i.e, the one who was forcefully bonded - in this case Sejin, cannot feel Ido's emotions but Ido can feel Sejin's emotions clearly.
That's why Ido was completely shocked after realising Sejin's emotions. It was extreme fear when they bonded and Ido then came to know every bit of what Sejin felt and that he was actually raped smh.
All this time he thought Sejin also acted like he loved Ido but never did after finding out his betrayal but, he then realised Sejin also loved him but not anymore. Sejin starts starving himself and gets extremely depressed and Ido started begging him to eat but Sejin was too depressed to care.
He did not visit Sejin Cuz he could tell Sejin did not like seeing him from his emotions. We only read Sejin's POV but his emotions are conflicted when all the while he was depressed.
Then finally when Sejin found out about his pregnancy, he did not know who's baby it was which made him fall into more despair and he decided to off himself.
But it was Ido's baby from the smell of pheromones, Ido could perceive it but Sejin was too out of it to notice.

Spoilers ahead:
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
From what I remember, when Ido bonded with Sejin, he experienced all of Sejin's emotions and memories retroactively. Ido realised the truth behind Sejin's behaviour and how terrified Sejin was of both him and his brother.
Ido then understood he messed up real bad but he didn't know how to proceed since he could feel all of Sejin's emotions post-bonding and was well aware that Sejin was depressed and extremely afraid of him now. Aside from the immense guilt, he thought Sejin would be better off not seeing him. (Rightfully so, cause Ido was a huge piece of shat)
That's why he only visited Sejin while he was asleep. Side note, Sejin wasn't the "gift" that Ido's hyung was supposed to find. It was supposed to be documents that were evidence of hyung's shady dealings/company mishandlings and such.
If Ido already knew what happened to Sejin when they bonded, he should’ve stayed with him. Although maybe he felt guilty, but Sejin suffered more when he’s lonely. Damn, you better level up your game in the present time, Ido. Talk to Sejin cuz my baby’s trying his best to communicate with you (as far as I remember)! Also, fuck Ido’s hyung! (╯°Д °)╯╧╧