He could have married everyone before the justification was said and it would have not changed a thing or felt out of place considering what the story has presented before and also considering the genre and tropes that the manga is following... it was quite unnecessary... also, the "legality" clearly was only brought up as a justification for future new additions to the harem group...
No, even "harem" series that won't allow real harems randomly add more girls.....
And honestly, I feel it isn't unnecessary, it just clarifies things and is giving readers a context while also being very short and unobtrusive.
And the issue would be if he married everyone at the end is that a legal harem is not the norm in the real world and even in a fantasy, it's not always allowed...
Too many fantasies also have no harems... so that legal clarification allows us to know that he can marry them.
It means it's not a sudden change, cause the other option is that only 1 of them can legally marry or he doesn't get married at all... and those are actually not uncommon either...
I read plenty of harem stories where the MC never marries his harem or he only had 1 legal wife....
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In fact, I think the opposite is worse.
When the MC is in a world where harems are legal or some other dude gets a harem and then the MC is in a normal relationship.
I'm thinking that the author should just not have the harems at all and just have the 2 person relationship just be a 2 person relationship...
THAT is the norm after all and no harem mention needs to exist...