Monkurochou

Masaya used to have a close friend in Momoko. One day, he caught sight of her pad under her skirt, and it terrified him. Since then, he hasn't been good with women, and he went to all boys' schools. However, he still ends up having to interact with girls, and it seems like everyone is way more sexually experienced than he is. Even when he has a sexual encounter with a girl who seems to like him, Lisa, its still unimpressive next to his friends' exploits. He has a hard time coping with discovering his sexuality and with the idea that his childhood friend Momoko is now a young woman with a sexual appetite of her own.

(Source: MangaHelpers)