In a country where privacy is a rare thing, and where young couples especially really have their own private space, a love hotel is the only option for a young couple to explore their relationship.
Many Japanese homes are shared by more then one person or have waffer thin walls, at least when sound is concerned. She is 19, probably living with her family, you were a young american soldier, you can bet you wouldnt have privacy, hence, The love hotel was the only option for making Marvin Gaye blush.