Imagine the storyline where the sexes are flipped. In other words, the lead character is a 30-something male, bored with life with a steady girlfriend, who decides to have a fling with a female high schooler. I'm not sure which culture would find this premise acceptable.
But for some reason, as long as the lead character is female, then it's OK and we go into a fantasy storyline.
As depicted in the story, the female lead is just bored of the mundane life she's leading and so goes looking for a hot, underaged pickup. Again, imagine if this was a male lead--any sympathies then?
Then we get into the psychologies of the various characters. I cannot fathom how the 30-something female lead acts like a 30 year old. She's more like a stunted 20 year old. Unhappy with life, she is not able to talk with her boyfriend about their relationship issues and instead decides to have a fling. Doesn't this sound more like what a high schooler or college couple would do?
As with many J-dramas these days, the original story came from a manga series that's geared towards young female audiences. Hence the main female lead feels like a 20 year old.