Ah, that's a good point! Hmm, but, remember: there were only human player characters at one point. Do you think they added Sera's romance plot after they decided to add race selection? Was there enough time to add a whole romance plot for non-humans? (No, seriously, I wanna know. What's the timeline for that sort of thing?)
Well, they had to change something after races were added, I believe. I mean, they already would have had to alter everyone to add these gates. In the past they've had relationships as blind to blood mages and races that wouldn't really make sense. They may have started everyone like that and decided gating made more sense later, after adding more races.
The one thing that makes me think Cullen or the last male are more likely, though, is that Vivienne is the likely second. Even if it's for "story reasons", I'm not sure they'd gate two females and no males, even if Iron Bull was originally planned as gated.