What was there to be offended by? The movie is, roughly summarized, about a 17-year old teenager, Chikako, who signs her body away for one year, to three men to use.
Certainly, there is a lot of uneasiness around teenager (14+) sex with older men in the U.S. But it is by no means pedophilia; by law in this country it is indeed illegal, as it commonly considered exploitation of a minor. In this movie, it is not really portrayed that way, because Chikako "willingly" signed her 17th year away to these men.
I put "willingly" in quotes, because the movie doesn't explain the background of why she's engaging in the contract. I'm assuming she didn't need the money for any particular purpose.
So what is her motivation? She doesn't appear connected to anyone, like her mother or to people at school. The father is not present in the movie. The back story is not clear, but I can imagine looking at the character on screen she was likely approached by somebody, and like many nihilistic and apathetic teenage girls, acquiesced.
Sexually, Chikako ("Hanako") seems game. Emotionally, she is entirely shut down. She doesn't care about her own name, or the names of the men who own her.
In some respects, watching a young, beautiful women be sexually exploited is arousing. Men intrinsically (biologically) desire such women. The men aren't cruel, but they treat her like their favorite toy. Chikako willfully submits with disinterest. In one part of the movie, they say she's like the "radio controlled airplane" and "10-speed bicycle" they never got as children.
What the film attempts to do is represent the audience as one of the men: At the beginning of the movie, the audience is unattached to the characters, but in a purely visual way gets to enjoy the sex as the men do. Later, as the characters develop, attachments and empathy develop. Both the audience and characters on screen, for lack of a better expression "fall in love." People--a large number, no doubt--who disagree with the premise of the movie will likely rate the movie rather low and leave in the middle. (These people likely hate pornography and erotic movies in general.) I really enjoyed the movie myself. I rated the movie highly, because I think it was a very original idea and very eloquent storytelling.