Ugh... Cassandra can't be both gender and race gated... right?
It's possible. Mr.Gaider already said that the reason they made her straight was because they were writing a sepcific type of romance for her. This means that they already gave priority to the storyline over making her acessible to as many players as possible. Thus, it is not impossible that they want to write a really, really specific type of romance and thus would restrict her further.
This is not, necessarely, a bad thing. It can be disappointing to not be able to romance her with the character you were hoping to create; I know I'll be sad if she is race-gated against humans; but it could also mean that her romance will be really deep.
I imagine it would be harder to write something deep and nuanced when the character can be romanced by humans, dwarves, elves and qunari of either gender. You'd have to adapt it to every type of Inquisitor which would leave little room for other types of romantic interaction.