It's guessing time
First thing first, trash manga is trash but this can be fun trash
Second thing, I'm gonna guess the gimmick of the manga is that he can only become Sage after fucking, due to Sage-time, as Japanese put it, happened after sex. And it will have a time limit, so he has to find females party members that willing to fuck him so can be strong. Looking at the cover, there is a good chance he will recruit the prostitute.