That novel is a contemporary (young adult) realistic fiction romance. (It's "one of the most talked-about romances in teen literature"–Kirkus Reviews).
Little, Brown, the publisher of Why We Broke Up classifies it as Realistic Fiction. This genre does not include fantasy, so Harry Potter, vampires and zombies are not included within it.
Good Books lists many young adult realistic fiction romances.
Study.com defines Realistic fiction:
Realistic fiction is a genre consisting of stories that could have actually occurred to people or animals in a believable setting. These stories resemble real life, and fictional characters within these stories react similarly to real people. Stories that are classified as realistic fiction have plots that highlight social or personal events or issues that mirror contemporary life, such as falling in love, marriage, finding a job, divorce, alcoholism, etc. They depict our world and our society.
Characteristics of realistic fiction include
- Realistic fiction stories tend to take place in the present or recent past.
- Characters are involved in events that could happen.
- Characters live in places that could be or are real.
- The characters seem like real people with real issues solved in a realistic way (so say goodbye to stories containing vampires, werewolves, sorcerers, dragons, zombies, etc.).
- The events portrayed in realistic fiction conjure questions that a reader could face in everyday life.