Yabu.
Its what teenagers do watch movies that are 15 and 18 rated.
The sex in the city movie doesn't really have anything more explicit than most under 15 movies.
I loved dirty dancing as a young kid never really noticed the sex scenes just loved the music and dancing.
And many other movies that I wasn't supposed to see.
I still had to wait many many years well after I left high school for my first shag.
She is unlikely to be corrupted by the movie, she has probably heard much worse from the easy girls at school.