maybe its my perverted imagination but Gyarus are usually delinquents that are defiant and do what they want.
So MCs crush is a "bad girl" now. Gyaru usually starts from a young age and MCs crush was a older chick already even when he was a tiny child...
what im trying to get at is she probably had a delinquent in her life that groomed her into a gyaru... and probably fucked her a lot...