Spoiler then:
This was a novel. In the novel, the king isn’t the previous king’s biological son but he was the Marquis’s biological son. The baby was born still birth iirc so the previous king switched the baby. ML (the king) doesn’t know that yet. And yes, colin is actually his biological brother.
Anyway, about marriage with nathanial. It was a political marriage. They married because it was something pushed on to them (very common at that period ig). They never love each other.
Fast forward, a coop happened, and they escaped….ML found out he isn’t legitimate but he started a rebellion iirc. Anyway they won. And this time he ascended the throne, and legitimately married Chris (mc). They have a child together later on.
Nathanial happily divorced ML, and went on to live happily with his lover knight that he known since they were kids. He will have baby with his partner.
In the novel, it was something he must do because of duty etc. He came from a powerful family so it was drilled into him. He gets with his lover after divorce. Anyway nathaniel family is the one that got mc and mc’s kid killed in the previous timeline novel. His dad is greedy and corrupted. He is the one behind the coop.

Reading the comments, and really wanna say something but I can’t because it would be spoiler. All I got to say is y’all judge too early and too harshly.