They should create interesting and diverse characters as they see fit instead of tailoring them to the demands of some fans. If they imagined Cassandra to be straight, they shouldn't change her because of the people who want to romance Cass but only play lesbian characters. Instead these people should realise that by being unflexible in their playstile, they will miss out on certain content, just like it's always been in this kind of RPG.
Characters are not more interesting and diverse because they are not bisexual. If a bisexual character is poorly written, their sexual orientation isn't the issue.
If Cassandra is a heterosexual love interest, so be it, but that does not make her more interesting, better written, or in any way "diverse."
If I, as a woman who wants to only play women in role playing games where female player characters are an option, decide that I don't want to play a male character in order to romance Cassandra, I'm not being "unflexible." I am not interested in being forced to play a male character in a fantasy game where I can be a fantasy race or shoot fire from my fingertips. I'll deal with not being able to romance Cassandra, but I would be disappointed.
I wish people would consider, when making the argument that gamers who are interested in same sex romances should just play out heterosexual romances with characters of the "correct" gender, that some of these gamers are told similar things in real life, about compulsory heterosexuality, and having to pretend to be heterosexual in a game that is supposed to be escapism is a depressing prospect.