Isn't it illegal for couples to live in a hotel together?

It's not illegal. Although couples are not husband and wife, there is no law that couples can't stay in hotels, so it is legal for couples to go out to stay in hotels, and there is nothing illegal. As long as they don't have any improper transactions or illegal acts, the police can't have any punishment.

Legal basis:

"Measures for the Administration of Public Security in Hotel Industry" Article 6 A hotel must register when receiving passengers' accommodation. At the time of registration, the passenger's identity documents shall be checked and truthfully registered according to the prescribed items.

To receive accommodation from overseas tourists, an accommodation registration form shall also be submitted to the local public security organ within 24 hours.

Unmarried adults can choose who to live with. Although they are not protected by law, the law does not interfere. Couples who are not registered to get married can stay in hotels, which is not illegal. However, you should carry your ID card with you for inspection by the public security organs.