What I find interesting about the fact that Cullen's romance is race-locked is what it says about his character and the romance more generally. Specifically, Cullen will only romance races he was familiar with within the circle. And in the context of the circle, Cullen has specifically always had power over elves and humans as a templar in a way he never has over qunari and dwarves.
From the characterisation we get, I wouldn't even assume this is a conscious decision. I do think Cullen generally loves the romanced inquisitior and sees them as an equal or even above him (Herald of Andraste style, the boys a believer if nothing else).
But I would say that Cullen feels himself to be very vulnerable - and I imagine has done his whole life, especially after the events at the ferelden circle. in DA2 that comes across as him literally beating up recruits in the woods; asserting his power through literal force. And while he's trying to overcome that and shows some remorse in inquisition, he also still feels that horrid vulnerability that I think really scares him. He says as much in the scene he tells you about Kinoch hold. It's one of the reasons he argues that he 'should be taking it' (lyrium) because he sees himself as weak.
And I think there's a subconscious element that he knows he has had power over elves and humans. He could theoretically control them like he did before; he's made strong minded elves and humans tranquil before after all. And I don't think he would do that to an inquisitor, but the point is that subconsciously the vulnerability he feels is protected in a way he has no proof it would be if he was with a dwarf or a qunari. he has been betrayed after all before; by the mages in Kinoch hold, by a warden he had a crush on who saved said mages even though he begged them not to, by Meredith.
Like I said. I think this is entirely subconscious. But I also think it's a more interesting angle to explore the 'whys' than simply 'they didn't have time to make the smaller and taller character models work with the desk scene'.
