You can skip romancing them and enjoy the main story or stay just friends in their 'romance' route. The bad endings occur based on ur choice with the witches and a certain quest involving ur other self.
I do recommend still exploring their route though as their events expands the world lore a bit, (Quill on witch hunters and mermaids, Thane on vamps, Arshem on fairies and Lyall on the fishing village and witches)
But they are entirely optional and you can just focus on the main plot if that is more your thing ^ ^