That's as apposed to previously bi orientations according to writer David Gaider.
"Dorian is gay—he is, in fact, the first fully gay character I've had the opportunity to write. It added an interesting dimension to his back story, considering he comes from a place where "perfection" is the face that every mage puts on and anything that smacks of deviancy is shameful and meant to be hidden. Dorian's refusal to play along with that façade is seen as stubborn and pointless by his family, which has contributed to his status as a pariah."
While nothing's been explicitly said, the idea that the character is 'fully gay' implies he's a no go for any potential lady-based romance options. That idea and phrasing's already caused trouble with comments on the interview discussing the character's orientation at length and one fan questioning the 'biphobic' nature of the statement: