I mean, even in that case freeing someone from slavery is pretty much the single greatest gift you can give, so there's still the implied reciprocity/quid pro quo, nightingale syndrome, etc. making it highly likely you're gonna get laid.
The hero slays the dragon and rescued the damsel in distress, he doesn't need to have a notary put in writing what he wants in return. Nowadays isekai is more like the hero asspulls a win over the dragon- that's-actually-a-loli, takes custody of the enslaved princess but retaining ownership and keeping her as property and fucks them both with little ethical concern because they totally want it.
Now that I think about it, I'm almost positive the "they're slaves but they love it so much they refuse to be freed and still want to fuck me" is at least tangentially related to WW2 comfort women denial and revisionism. It makes too much sense.
"The sex slaves enjoy it so much they don't even ask for their freedom, and anyways they receive the honor of sheathing glorious 4 inch samurai sword penis from honorable samurai genes, they should be more greatful, they wouldn't know what to do with their freedom in the first place."