I'd argue it started with Humanism in the Renaissance and matured in Protestantism before the Enlightenment took it up, but this isn't really the place for a history debate.
Speaking as someone who wrote her Honours thesis on Petrach and her (unfinished) Ph.D. on Ficino, I don't know that I'd agree with you there. But you're right, it's not the appropriate place to debate the subject
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Really, the "white" part of that came into vogue as discrimination a bit later than we all suspect; for a pretty long time period, it wasn't as big a deal; there were white slaves and black slaves. Racism picked up when the church (ironically enough, the Inquisition) started trying to drive the "Maures" (or Moors) out of Europe.
Yeah, skin-colour based racism was a late invention in Europe. It doesn't really become a universal ideal until the Transatlantic slave trade makes it expedient to believe that Africans aren't proper humans.
So, uh ... how about those romances?
In an attempt to drag us back on topic: what unavailable character(s) do you wish you could have romanced in previous games?