I am laughing so hard right now. So many comments bashing this and yet YOU ALL read it. The cover alone can tell you the main couple. Hell the first few pages can tell you that and yet you read it and are now complaining about it.
If you look at history you can see that this is normal. In fact Bunchi is being a pure gentleman compared to others. He's not having sex with her, he's not touching her breasts or any other inappropriate place, they haven't undressed at all (taking his hat off doesn't count), he's not picturing her naked, and he's not making innuedo. The most he did was kiss her. That's it. Go into the hentai section of ANY manga site and you will find at least 1-3 mangas on the first page that will have a girl Himeko's age doing things like sex, masturbation, etc. and NO ONE bats an eyelash at it.
An older guy sweetly kissing his younger fiancee, yeah no, that seems to get everyone up in arms. *rolls eyes*
On the historical part. Please look up any society in the last two hundred years or more and you will see that this is normal. I'm not kidding you. Also be glad that they have romance. In the Taisho period, marriages were not arranged for romance but to continue the line. In fact, romance was considered essentially taboo during that time period of Japan. According to the wikipedia page on Japanese marriage
"Boys and girls were separated in schools, in cinemas, and at social gatherings. Colleagues who began a romantic relationship could be dismissed, and during the Second World War traveling couples could be arrested. Parents sometimes staged an arranged marriage to legitimize a "love match," but many others resulted in separation and sometimes suicide. Love was thought to be inessential to marriage."
It wasn't until after WWII that they made a law saying you could marry for love. So try not to scoff or put down this love because the age difference is big. It wouldn't be considered "wrong" in society now if she was 18 (36 for him I think) because the age difference would be the same no matter how old she is.