The Dreaded 69

Just food for thought...
Why is it when you ask kids for their favorite number someone always shouts out "69!"??? Oh yeah, because they are 16 years old. I remember being in high school and everyone would snicker when the answer to a problem would be 69, but these kids truly would be proud to have 69 tattooed on their foreheads! The worst part is my first few years of teaching the boys would be the ones to yell it out, now it's the girls!!! Don't they get enough sex-ed in bio???