I'll never understand what's so scandalous about a "young" (like here, 12yo with 19yo) age-gap couple when no one ever bats an eye about an "old" (say, a 30yo with a 40yo) age-gap couple. I would understand the backlash if the story depicted the MC switching from young girl to young girl every year because he wants them young, that's disgusting and I'd be the first one to pick up a pitch and a fork in protest... but as far as I can see, it's portrayed as genuine love on Yuka's part, so are you all saying the girl shouldn't try to be happy because "nuh-huh, FBI!!11!!1!" ????
Not to mention this is fiction: not that it would be a problem if something similar happened IRL, but you could at least try to excuse the thing as "it's just fiction" instead... which makes me believe this insistence on the age-gap is more concerning for you lot instead.
You misunderstood so much.
First, as someone pointed above. 40 and 30 year olds are already adults, with multiple years of experience, they might as well have a similar social standing. A 12yo and 19yo, are VERY different. Maturity is not the same for either. How old are you? That line of thinking would not be the same if you were older, so I am guessing you're still a teen. If you aren't, but an adult... I implore you to please think about this situation and why society as a whole frowns upon this (please, DO NOT bring history into this equation). The age dynamics here are extremely important, this isn't just about age gap, but WHERE that age gap exists.
The issue is not about the guy cycling through girls. It's about the guy have the responsibility of an adult and how to deal with children.
Yuka's love might be genuine, but this problem isn't about how SHE feels. In fact, taking her side is being nïeve. This isn't about wanting to destroy her love, but to bring some sense into an adult irresponsible enough to lead a child on (so far). Besides, no matter how genuine, you can still do bad things... and Yuka did something wrong even if her intentions were not bad.
Just because this is fiction doesn't mean discussion can't arise from this. The point of fiction to tell a story, and a story has a message. The author is trying to convey something with each part of the story. And so far, they have shown how bad obsession is, and to a lesser degree how this guy is leading women on. Are we supposed to not discuss what the story is doing? Part of the enjoyment of stories isn't just to read them, but to understand them, and to involve ourselves in the plot... not necessarily personally, but critically. And communities exist around them, because it is an aspect of socialization. We're looking at two fictional characters who are portrayed as if they believe their story is real, not fiction. We see us in them. And it is valid to consider what the story is saying as it was real given the context, especially when there's a message you can take from this.
Contextualize this story in reality, and see how you feel about it. Suddenly, there's a point for such a discussion. Fiction or not, people learn from stories too.