Yet another thorough and interesting review. You've understood most parts of MoR:s lore surrounding slaves. And yes – comparing this to the roman era would be wrong. Slavery in Ikaanos evolved through poverty and chaos - as a way to protect your family and save yourself from starvation in the aftermath of the severance. The concept evolved from a highly evolved society, and even if 300 years have passed, some "civilized understanding" of a contract between master-slave still exists in today's raanic society.
The next branch of updates (0.8.X) will focus on slaves, and I intend to flesh out the lore surrounding contracts and the social status of servants. This might change some parts of what you mentioned in your post – including the market for slaves. I must admit that the slave agent's location in Town hall is a sort of placeholder until I develop Crystal Hills and the seedy Westside slums.
With that said, it is still perfectly okay to use your slave for sexual purposes, in the eyes of the law, even if some of the girls you meet in Ikaanos might see their contract as a "maid's bond" – never thinking that their apparently kind master would use them sexually. See it a combo of cute ignorance and the civilized remnants from olden days.
I'll structure this more properly down the road - after getting MoR:s framework into a not-so-embarrasing and relatively bugfree state. But you've given me some interesting ideas!