@jokerxhisoka
No, the MC surely could do it with normal girls, too.
But that would require the author to write a story about how the MC gets to know the beauties, how he gets closer to them, how the girls develop feelings for the MC and all such things.
And that for EVERY girl individually and withour repetition of the same things.
The slave and brothel version on the other hand is far easier:
1. MC went to get slaves / have fun in the borthel -> He knows the girls.
2. MC does something good for the girls like buying them while a bad man aimed for them as slaves or healing some illnesses they get due to thei jobs in the brother -> They have a reason to like the MC.
3. As slaves they have to do whatever the master tells them anyway / By working in a brothel they are paid for the desired action in the first place. -> They have a reason to do it with the MC.
So as you can see it is simply the author taking the easier way because he does not have the ability / time / interest to write a deeper rooted development for the girls.