Love Junkies

Love Junkies (恋愛ジャンキー) is an hentai comedy manga written and illustrated by Kyo Hatsuki.

Description copied from Wikipedia:

Sakibara Eitaro is a 22 year old virgin. He's desperately trying to get laid - and that happens right at the beginning of the story with a girl named Maiko.

After that, Eitaro's relationship with girls change suddenly: he starts feeling more secure of himself and sex, and meet lots of girls. Three of these girls will have a very important part on his life (Ninomiya Emu, Ide Miho and Jii Shinako) and his love adventures revolves around those and other women.