Peter Grill and the Philosopher's Time

Peter Grill to Kenja no Jikan (ピーター・グリルと賢者の時間) is a manga series by Daisuke Hiyama. It is serialized in Monthly Action.

Plot
Peter Grill is known throughout the land as the world's strongest fighter. He has a girlfriend who he's steadily trying to get closer with. Life is good for him. Until he realizes that dozens of women, who want to bear strong children, are all vying for his seed.