Romances seem to be quite time and resource consuming content. Why would they want to place *double* restrictions- gender and race- on them, to the point where only a tiny fraction of the audience ends up experiencing content that took lots of effort and resources to create?
Cutscenes for romantic content seem to swallow up a big portion of the effort and ressources. Given that each race/gender-combination has their own model, requiring additional effort for the cut scene, double restrictions make sense for keeping the budget.
Say they had budget for 32 cut scenes with different models. They either could spread this over 4 characters being LI options to everybody (4 characters * 4 races * 2 genders = 32), or they could say "Sera is only available for for half of the model types, and Cullen is only available for the other half, allowing us to have more than 4 characters as LI option.
This is only a very simplified calculation of my own reasoning, but I would be surprised if budgeting the scenes wasn't an aspect in the restriction.