Nagashima Chousuke - The Otaku in 10,000 B.C. 1 (English) - Hentai Manga

A hopeless otaku named Ota. Unlucky at love and nothing going his way, Ota's world is turned over when he’s magically transported into a land that time forgot. Surrounded by nurturing, big breasted cavewomen with seductive banging bodies, Ota’s going to have his hands full teaching them a bunch of firsts in the history of sex!