Why does the Torah give the girl such a lengthy description "Na'arah Besulah Me'orasah"?
1.
Sanhedrin, 66b: "Na'arah" - 've'Lo Bogeres'; "Besulah" - 've'Lo Be'ulah'; "Me'orasah" - 've'Lo Nesu'ah'. If the girl is a Bogeres,
1
a Be'ulah
2
(not a virgin) or married, or where her father has handed her over to the Sheluchei ha'Ba'al
3
she is not subject to stoning.
Rashi (in Kidushin, 9): Because a Be'ulah is Chayav Chenek like a married women (as is a Bogeres - even if she was a Na'arah at the time of intimacy - Kesuvos 45a).