I am sure that our companions' sexual orientation in this game is dependent on their backstories and personalities, something akin to that of DAO. This will not be like DA2 where the romances are Hawkesexual.
Morrigan is straight because she was raised by Flemeth, a woman who viewed sex as a tool, a means to an end and had "company' in her house to satiate her desires.
Alistair is straight because he was brought up in a Chantry, among the Templars where traditional sexual roles were prevalent.
Leliana and Zevran are bisexuals because one is a bard and the other an assassin. People in this line of work would do well to be open minded in their sexual options.
Sten is non-romanceable because in Qunari culture there is no such thing as flirting or love. There is breeding program to produce offspring and the resulting progeny is raised by the society, so to speak. Hence, one could say that he is asexual.
Which brings us to Cassandra Pentaghast.
If we used similar train of logic, one could say that she is straight because she was never shown to have any romantic feelings for women. We only know of her dalliance with Regalyan, a human mage.
That being said, Cassandra is someone who values someone on their merit which means if the Inquisitor proved himself to be a honorable male, be it an elf or dwarf or Qunari or human, Cassandra would not have a problem romancing them.
At the same time, Cassandra is a butch female. She certainly looks the part and seems to behave as one. Now it might be because of her training as a hardcore warrior and Seeker but it is a known fact that in the generic sense, butch females tend to be either lesbians or bisexuals.
All in all, we need more data, more information from the developers to effectively say if Cassandra Pentaghast is a straight female or a bisexual female.