To get it out of the way, even though having consenting sex with a person of Sayu's age would be technically legal in Japan, and in fact most countries, it's still a social suicide, especially under those circumstances. That alone is one reason why avoiding sex with her is a big factor. That being said, it still is a big problem in Japan. However, there are also a lot of cases where high school part-time prostitutes have pulled the rug from under their customers. It doesn't help that prostitution is illegal in Japan! If a judge decides that receiving accommodation in exchange for sex equals to receiving money in exchange for sex, you are done for.
If it's an adult woman living with you, unless the woman reports you to the police, it's nobody's business. You can be viewed as a couple. No child/youth protection laws protect adults. However, allowing some stranger to live in your apartment in exchange for sex would seem like a huge security risk in many ways if you ask me.