Should i be concerned about my son?

I'm so embarrassed. I had to see my 12 year old sons principle today. Apparently they found scribbling at the back of his excersise book and the sentence 'I WANNA FUCK GIRLS' I am mortified. I didn't even know boys this age knew or was interested in such things. Am I doing something wrong as a parent? What should I say toy son about this? This is the first time I have knowledge of him being interested in anything sexual.