Its because they are nasty freak's and thinking of perverter things. These type of Manga are so shit its always about the Author being the MC of their manga.Man the self projection of the author in this manga is insane with the Japanese cliche textbook wimpy stuttering protagonist who acts like he has never touched grass in his whole life ...
But seriously though why can't they act normal ? And always stutter when women are around and and blush 24×7 even to the slightest touch everytime no matter if they're girlfriend or crush