18+ Koushaku Chakunan Koushoku Monogatari: Isekai Harem Eiyuu Senki

A Japanese salary man was reincarnated in a different world with swords and magic. He reincarnated into the family of a powerful noble. One day when he became twelve years old, he was suddenly summoned by his father.

"I do not mind whether it is a woman in the town or a maid in our house, learn about women."

Having gained the permission of his father, he uses his power as a noble to have his way with women.

(Source: Shousetsuka ni Narou, translated)