@Xalef @matrix it's a pretty meh isekai, the dungeon crawling is especially brutally featureless, even worse than The Great Cleric's, but it has its moments and I'd still read it.
The sex slave on the cover is a dwarf who people in the world look at and see as a doddering 90-year-old rickety old lady--because her ears are thin, and dwarf ears thin with age. But she's actually a teenager with a genetic defect that makes everyone think the MC is a pervert with a grandmother fetish (this part isn't emphasized by the story).
Races in this world also have special classes that are unique to each race. Humanity's racial is "sex fiend", a class with sex-based skills, and the in-world explanation that everyone accepts is "well, humans are sex-obsessed like that."
The MC's cheat is manipulating classes and sharing skills with party members, so although in-world a sex fiend would have to make a living like that somehow, the MC can turn it on and off. Mainly, he makes his slaves overpowered.
A big trope of Japanese light novels (after harems and simp MCs) is 'humorous misunderstandings', where the MC has what he thinks is a perfectly normal day and then a POV chapter reveals that people around the MC are close to dying of shock or kneeling in worship or scared to death of the actually crazy shit the MC's pulled off. In this series the humorous misunderstandings are limited to the MC never explaining his cheat and dodging questions about it, mostly from the sex slave dwarf.