This is like Anders' situation though. In previous games both he and Cullen showed interest only in PC and people of the opposite gender
So unless it means that our male PCs were ugly as frag it seems like a retcon. And I don't like retcons
Oh, and all this "Thedas-is-100%-bi" is kind of weak since we rarely if ever see bisexual relationship outside of the party. If it was that common we would surely see such families, couples in great amounts, wouldn't we?
Unless of course bioware are simply doing a bad job at showing it in the game, novels and comics...
Where to begin?
Okay, so Anders wasn't a romance option in DA: A. Yes, he made a few off the cuff banter lines about "pretty women", but that in no way indicates that his bisexuality in DA 2 is a retcon. His writer has already stated that Anders was conceived as a bisexual character back in DA: A development.
Cullen may or may not be bisexual. He might have expressed a crush on a female mage 10 years earlier, but only if you played as a female mage. The same way that Fenris maybe hooking up Isabela isn't enough evidence to tell us what his sexuality is, this isn't enough for Cullen. We know he liked women. If were to be gay, then that would read as a retcon to me. However, if it turns out that he likes both women and men, then that's not a retcon. I'll put it this way. If you and I go out for dinner and I order the steak, you can assume that I like steak. If, 10 years later, we go out for dinner again, but this time I order the chicken, it's not a "retcon". It's not a drastic change in my personality. It's just that I have always liked both steak and chicken, but you only had evidence about me liking steak.
And what, exactly, is a "bisexual relationship"? Is a bisexual man in a relationship with a straight woman a "bisexual relationship"? Because we've seen, literally, dozens of man/woman relationships and I can't think of a single one (except maybe Oghren) who indicated that they are straight and not bisexual. People see a man and woman in a relationship and assume that both people are straight. Or two men in a relationship and assume that both are gay. But that's not necessarily the case. Now, do I think that everyone in Thedas is bisexual? No, I don't. However, can I safely say that there is a chance that there are more bisexuals in Thedas than (at least those who admit it) in real life? Yeah, I might be able to make that statement. So unless you are advocating that every NPC who is in a relationship has to openly state their sexuality in some way ("Inquisitor, please save my wife! I mean, I also fancy men, but I really like her best!"), how exactly do you want Bioware to "show" bisexuality? It seems silly to me.