I doubt David will approach the subject of Cassandra's sexuality. He's never felt the need to explain why Morrigan, Viconia, Bastila, or any other romance he's written is straight, why would he start doing so?
Apologies, not my intention to sound douchy about this at all, but I dont understand this. How does one explain why someone is of a certain sexuality?
I happen to be Bisexual, but other than the fact I rather enjoy both the male and female form, I don't really know how I would explain WHY I am Bi. I simply am?
As Gaider is responsible for writing Cassandra(and the other characters you noted) and as such gave birth to her personality, beliefs and convictions so if he says she is straight why would he need to explain it? I am sorry, it's just sounds odd to me.
Again, not wanting to be douchy, it just stood out to me and I was curious.