Why slavery? Let's see, I can work the fields myself (no, it sucks), pay out the ass to hire people to work it for me (and they'll keep demanding raises, breaks, etc.), or I can just buy a bunch of slaves, let them do the shit jobs, and beat them when they complain. If they die of exhaustion, heat stroke, malnutrition, or beatings, I can go out and buy/capture a new one.
Sexslaves? Train 'em properly so they have no skills outside of pleasing me and make sure they know they're useless; their only means of survival is keeping me happy, running away means they'll just die of ignorance in the wilderness or get caught and given an even worse fate. Sexslaves are exclusive to you, so you don't have to worry about diseases or raising other men's children.
If slavery was financially unsustainable or impractical, WHY DID EVERY CIVILIZATION ON EARTH PRACTICE SLAVERY AT ONE POINT OR ANOTHER? Why does it continue today in Africa and the Middle East, or as sweatshop labor just about everywhere?
Slavery exists because you can treat people like property and make a profit off of it. Slavery never ended because it didn't make financial sense, it only ended because of moral indignation or slave revolts.