Mia's not too unrealistic beyond the fact most girls who act that way grow out of it by the end of high school, even if they don't lose their virginity. I did a tour of Europe with a bunch of teens from across the US and we had this one 14yo girl on my bus who talked about sex all the fucking time.
Halfway through the trip she stopped talking about sex. Everyone knew she'd lost her virginity.
What happens is they imagine sex to be something totally amazingly incredible and it's all the can think about. Then they have sex and it can't measure up to their completely unrealistic fantasies and it brings them back to reality. Mia apparently went a different route, she started using sex toys, then loved the feelings so much she's gotten obsessed with sex because of that.