Thank you so much for your lovely and kind comment!
To answer your question, the idea behind "villain romances" is that you can romance any of the five characters no matter which side of the conflict you land on.
So for instance, you can side entirely with humanity but still romance Maeve. She'd be acting as a villain to you on this route (and you'd be playing the role of villain from her perspective). Same goes for romancing Az'Lean while siding with the fae and actively deceiving/betraying him, etc.
Perhaps "antagonist romances" would be the more correct term, but I prefer "villain" for the fairy tale-esque flair. Of course, just because you can romance a character, doesn't mean it'll end well. Villains don't usually get a happy ending, after all...