He opens his own eyes to sexual matters.

Natsu considered it for a moment, before he rummaged around in his pocket, finding the pen he used to sign mission contracts, and wrote, "Natsu Dragneel will understand what the big deal is regarding sex."

Of course, he did know what sex was, more or less, he just never had that much interest in it, but with a magic notebook regarding it, he had his curiousity piqued. Nothing seemed to happen however. After a few minutes, he scoffed and said, "Guess this is a dud..."

He was about to toss it aside, when one of the Guild females left the Hall, and he noticed them.