18+ Teinengo wa Isekai de Taneuma Seikatsu

Following his retirement party, a drunk Kyouhei Maeda falls asleep on a park bench. When he opens his eyes he finds himself looking up at a beautiful young calling him by a strange name. Is this a drunken dream, or could it be one of those popular isekai novels?!

Reborn as the fourth son of a Countess, Ludwig von Freudenstadt must navigate a world where women hold all the power! Unable to use magic, men are only valued for their ability as "Studs." The best studs produce the most powerful offspring, and Ludwig is quickly approaching his first breeding session. When his partners and their daughters all begin to exhibit strange magical qualities Lutz suddenly finds himself surrounded by women! With genes wanted by even the royal family, and a whole other world's worth of knowledge, can Lutz even manage to live his desired peaceful life?!

(Source: MU)