Ore no Level Up ga Okashii!: Dekiru Otoko no Isekai Tensei

Zarmen, an adventurer who can't level up decides to retire from the adventurer business. Furthermore, in order to graduate from virginity, he decided to splurge on all the assets that he had diligently saved up since his reincarnation at a high-class brothel. The woman who appeared in front of him was the finest woman he would ever have known. Thanks to her hospitality, Zarmen fully enjoys his first time, but at that moment, his ears hear a mysterious announcement...

[Impregnation confirmed.]
[Level up!]

"Does this mean that the more 'seeding' I do, the stronger I get?"

An extraordinary implantable foreign world adventure fantasy created by a bottom adventurer who has been at level 1 for many years levels up through sex!

(Source: Frontier Works, translated)