No, they weren't "technically straight" just because you can't play as both a male and female in the same playthrough. That's not how bisexuality works. A bisexual woman married to a man isn't "technically straight". Similarly, a bisexual romance with an opposite sex PC isn't "technically straight".
I think they meant that from an ingame perspective, you don't know they're bi because you can't play both Hawkes at once, whereas we meta-know it when we play. I could be wrong though.
I can't see any lesbian fans of Cass being upset that straight women are being represented with Cassandra. I mainly see bi/lesbian Cass fans a little hurt and very annoyed by the complaining about her looks that, I stress again, a few straight men have been doing. Some of those posters are so damned loud, unfortunately.
I'm in the latter category. I think it's really cool that they have someone like her as a straight option, and to me it shows how much some people need to grow up. I was fairly disappointed at first, partly because it took almost til the end of the game after using every flirt option for her to reject my Quizzy, and partly for an odd personal coincidence. I'm still a bit disappointed, but I no longer think they should use the DA2 route for romances (which has good and bad points) and am OK with her being off-limits. Just annoyed that I like no one else 