Slave harem, slave love-interest, and slave-owner MC in general. It just feels kind of off-putting and tends to whitewash how bad actual slavery was and still is. I still read some stories with this, but it's just something I don't really feel like I want to support. The excuse MC's give of how "I don't treat my slaves bad," or, "I can't release them because they're <insert marginalized group here> and will be abused, recaptured, or killed if found wandering free," read pretty similar to the some of the dialogue that groups of racists and historical revisionists use to explain why they think slavery wasn't that bad. American slave owners often 'claimed' excuses of how they were protecting, educating, or improving the lives of their slaves as a justification for continued abuse.
Another consideration is that this trope seems common in Japanese LN, which seems to inspire some English language works. Japan has a history of enforcing sexual slavery on civilian populations captured during war as recent as World War Two, with some such persons still alive who demand acknowledgement and apology from Japan. Generally speaking, Japan tries ignore, minimize, or decline to acknowledge these former sexual slaves (or their descendants on their behalf), and thinks it's generally fine to put out novels glorifying sexual slavery. ¯\_(ツ)_/¯