I don't get it – why T.F. almost every fantasy manga (Isekai sins the most in this case) portrays prostitution (!!!) like the most normal thing in the world?! 
Are the authors high or do they not understand what it is?
This tendency to ruin such a fine worldbuilding annoys me to no end!!!
The attitude of these authors towards slavery I won't even mention! ...(cough)... Shield Hero... (cough)...
I completely understand if these unforgivable crimes against human beings (or any sentient being, for that matter) were only part of a setting and the protagonist would react adversely towards them...
But he (most of the time) or she (in fantasy) welcomes them as something good and useful!

Since when did sociopathy became a norm in fiction?
(After reading comments)
Boys, boys, relax! I completely understand your claims of "prostitution being an oldest job on the planet". I really do.
BUT! You are not on the service end, so you have no clue what this "job" actually is. And what it does to women.
And you can't know. You aren't women.
But imagine just for a second, that you are. And you're taking this "job". Would you still look at it as a "normal profession"? Would you feel on equal footing with a craftsman, a blacksmith, engineer, architect, doctor, teacher, academy professor? Would you be able to proudly say to your friends and/or family, that you're a prositute?
There's a reason why this "traditional profession" is an exploitation of human despair. Nobody choses prostitution for their curriculum out of free will.

Are the authors high or do they not understand what it is?
This tendency to ruin such a fine worldbuilding annoys me to no end!!!

The attitude of these authors towards slavery I won't even mention! ...(cough)... Shield Hero... (cough)...
I completely understand if these unforgivable crimes against human beings (or any sentient being, for that matter) were only part of a setting and the protagonist would react adversely towards them...
But he (most of the time) or she (in fantasy) welcomes them as something good and useful!

Since when did sociopathy became a norm in fiction?
(After reading comments)Boys, boys, relax! I completely understand your claims of "prostitution being an oldest job on the planet". I really do.
BUT! You are not on the service end, so you have no clue what this "job" actually is. And what it does to women.
And you can't know. You aren't women.
But imagine just for a second, that you are. And you're taking this "job". Would you still look at it as a "normal profession"? Would you feel on equal footing with a craftsman, a blacksmith, engineer, architect, doctor, teacher, academy professor? Would you be able to proudly say to your friends and/or family, that you're a prositute?
There's a reason why this "traditional profession" is an exploitation of human despair. Nobody choses prostitution for their curriculum out of free will.