Somehow, Crystal (1981)

While her boyfriend Jun'ichi is out of town, college student and part-time model Yuri passes the time in Tokyo by shopping for luxury products, visiting affluent neighborhoods, eating expensive food, and seeking new kinds of entertainment. At a dance club she meets Masataka, to whom she describes her relatively frictionless life as "crystal". Yuri has a sexual encounter with Masataka that she enjoys but finds less satisfying than her experiences with Jun'ichi. When Jun'ichi returns, Yuri learns that he also was unfaithful during his trip, but she reflects on her financial independence and decides that staying with Jun'ichi is the best fit for her crystal lifestyle. Adaptation novel by Yasuo Tanaka.