Damn, I'm sad now. I really liked this manga, maybe I'll read up until that point and duck out when it becomes too much.
Even though it's set in a vague time period, only people of a certain status or those being used as trade would marry at a young age. Putting that aside, given that this is neither based off a true story, nor does it follow many of the other rites and rituals of the vague era is was set in, and is written in the 21st century, there wasn't a need to to portray a romantic relationship between a 15 year old and a 20/21 year old 😑, it's as simple as that. I really like the set up and dynamic of the characters but this ain't it chief. Wait till she's in her twenties when her frontal lobe has developed enough for her to be fully conscious of her relationship and the decisions that are made (maybe gain experience), even if she is a 30 year old with a brain of a child.
Vice versa, let's say her brain is fully developed and using her previous 30 years experience, she manipulated the 19 year old king. That's still not ok.
TL;DR just because it's a part of a culture, doesn't make it ok