User blog comment:Jopjopjop/Volume 18 Spoilers/@comment-74.110.108.150-20140910120029

I was thinking, what if Issei's kiss with Irina when they were kids was the main reason for his pervertedness, 'cause he thought Irina was a boy at the time, and it so traumatized him that he blocked it from his memory and actively sought out girls to make up for it, and when he realizes the truth he will become less perverted as a result.