"You're equivocating! You said 'I have met hundreds of people who insisted that their slaves were happy and every single one of them was wrong', which implies their slaves were actually unhappy, and that this was bad. But then you said 'happiness, any word for it, is not really capturing what makes a good life', which is implying that it's not the absence of happiness that's bad! Your argument only holds if it's 'I have met slaves who claimed to be happy and, from my position of oh-so-superior knowledge of what people need, decided that they didn't have good lives'.
"Look, the reason you shouldn't trust someone who says 'my slaves are happy' is because they have ulterior motives. They have every incentive to lie or delude themselves so, as a general rule, anyone who keeps someone in involuntary servitude and then claims they're happy should be disregarded. Centuries ago, my world had ubiquitous slavery. Then the wealthy nations outlawed it and used their influence to suppress it everywhere else. I am certain that this outlawing was one of the best things that ever happened, and improved millions of lives.
"However, at the same time, I know people who voluntarily choose to serve someone else - often a romantic partner - in the role of a slave. I say 'choose' because they decide that this is what they want going in and have the power to dissolve the relationship at any time. They generally claim to be happy, and I see no reason to tell them that they're wrong about their own desires. It's not what everyone wants - I'm pretty sure it's not what I'd want - but it is what some people want, and I respect that.
"For someone who loves liberty, you sure are quick to scorn people when they use it for things you disapprove of."