Erectile Function and Sexual Activity Are Declining in the Younger Generation: Results from a National Survey in Japan by Akira Tsujimura, Shinichiro Fukuhara, Koji Chiba, Tsuyoshi Yoshizawa, Hikaru Tomoe, Masato Shirai, Kazunori Kimura, Eiji Kikuchi, Eri Maeda, Yoshikazu Sato, Atsushi Nagai, Koichi Nagao, Haruaki Sasaki