Does a unique premise outweigh a flat story?

Of Fire and Stars is the sort of book I wanted to like more than I actually did.  An Onion AV Club writer listed it as something she was reading and piqued my interest, as she noted that the story was a typical fantasy tale made more complex by its protagonist falling in love not with the prince she has been promised to in an arranged marriage, but his sister.

The book treats the romance between Denna, the princess destined to marry Mare’s brother, and Mare herself, as out of the ordinary only inasmuch as it complicates their countries’ alliance, to the narrative’s credit and detriment.

On the one hand, I loved that the gay relationship is portrayed as nothing more than falling in love with anyone else would be; that Denna has fallen for a woman is treated as essentially a non-issue, and the only hint that straight relationships are still seen as the norm comes from Denna’s misreading another lesbian relationship as friendship. She has to hide – both her relationship with Mare for the sake of political alliances, and her power to manipulate the elements, as she has magical gifts in a culture that rejects them (which isn’t a strong enough allegorical substitute for LGBTQ persecution)- but not for the mere fact of loving a woman.

Which is the exact sort of treatment I would like to see more gay relationships given, if only the rest of the book could support it. The gay relationship is the most interesting part of the book, the only twist on an otherwise conventional fantasy tale. If the central relationship were heterosexual, the book would read as George RR Martin lite. The murder mystery is fairly obvious, and the characters are somewhat flat; the chapters alternate from Mare and Denna’s perspectives and despite their dissimilar types I often had to go back to check who was narrating.  Moreover, I never felt the two falling in love. Their connection is frequently described but never felt tangible on the page, at least to me.

I want more stories where a gay central relationship is a non-issue, but the failure to explore same-sex relationships here felt like a missed opportunity.