Kigenzen 10000-nen no Ota

Even though this story is from the past, things like big breasted milfs are timeless! FAKKU is back with a prehistoric tale that was almost lost to time, The Otaku in 10,000 B.C. by Nagashima Chousuke

Our first Isekai release is centered around a hopeless otaku named Ota. Unlucky at love and with 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!

(Source: Fakku)