It still needs to not be treated casually. Like drinking a cup of tea in the morning. And if they are so openminded and living a state of brotherhood and unity, then that in itself is worth talking about too. It's unique and significant.
At least with a mage, for example, you can glean from a lot of background on how Cass is becoming more openminded. There's a lot of material for story here, both indirect and directly. While the whole game itself is about Chantry and mage coexistence, making the relationship a symbolic step forward on a bigger issue.
I think Iron Bull and Dorian's relationship plays out better on this racial level, because these two in fact are about struggling with their origins.. and what better way to show they're evolving by hooking up.
See, I'm fine with this. Addressing those differences, discussing them, etc. is all well and good and something I'd love to see more of. I love how, as an elf in DAO, you can call Leliana out on her unintentional prejudices. I love that you can ask Alistair if he's okay with you being a dwarf/elf and his explanation. I love that Fenris and Merrill are surprised that you, a human, would be interested in a relationship with them. I just don't understand people acting like it'd OOC if those relationships happened sans those conversations.
And about a mage being a good fit narratively... everybody takes something different from their playthrough. There's no right or wrong way to do it. Some people swear by the Cousland origin or mage Hawke for their story relevance. Others like the idea of Amell + Hawke + Cullen romance, or rolling a Dalish Warden, romancing Merrill in DA2, and playing yet another Dalish in DAI because they're sort of interconnected with the Eluvians and elven lore and all that.
I think mage + Cassandra makes a great story, but I don't think it's the only great story.