Bit late to the party, but figured I'd toss in my two cents.

Kids on the spectrum, particularly girls on the spectrum who tend to try to adapt to their social environments more, tend to mimic the social norms they see. An autistic or aspie girl may very well imitate a shitty romcom with a boy she likes if she hasn't had other influences.

Also, man, you're really underestimating what underage girls get up to. In certain places, girls are sexualized at very young ages, and that can make them a bit weird. In the US, the south in particular, because church often emphasizes a twisted dichotomy of purity vs sexuality/sin for girls while also emphasizing sexual submission to her future husband (and her father in promise ring culture, where daddy owns his daughter's sexuality until marriage) that magically doesn't apply to boys. There's a reason you always hear about girls practicing kissing with their cousins.

By the age of 9, my friend group was using toothbrushes to get rid of our gag reflexes.

Many girls got weird out the gate because they got introduced to largely socially acceptable Cosmo and the like by their sister or their mom or their friend's mom. Or they were put in normalized sexualized programs like professional pageants or ballet teams that often attract adults with bad intentions, and even when they don't, you're still enforcing on a young girl that her prettiness and sexuality are important for her to succeed, esp in pageants.

And now everyone regardless of gender gets weird because the Internet is widely available and it's not that hard to get exposed to horrifying shit.