I think this is boiling down to the same issue that romances always run into - the notion that NPC sexuality is somehow
fixed and real outside of the game.
Now some characters can have their sexuality be a defining part of their character - it's an element of what makes them who they are from a storytelling perspective. (Isabella, Zevran.) But some characters can have a kind of zero-state sexuality in regards to story. That means their sexuality isn't linked to their personality as vitally and as such they could be gay or straight and it wont change how they are in the story that is told.
In this case, their sexuality is in a zero-state. It's undefined until the PC chooses to romance them. Then the game goes "the PC wants to romance this character, for this story they will be made Gay/Straight based on the PC gender." From the player's perspective, the character has always been gay/straight. But from a metagame perspective they could be either but not both.
I suspect that this may be the case with DA2 as both the OXM and PC Gamer reviews have kind of implied that all LIs are romanceable regardless of gender without explicitly stating it.
This doesn't technically mean that all LIs are bi - but that all LIs are romanceable and their sexuality is defined by your choices. Just as your choices in the game doesn't mean all the choices happen and are objectively canon, the NPC's sexuality is not canon outside of your playthrough. Just because Merrill starts having a relationship with your fem Hawke doesn't mean she's gay in someone else's game of DA2.
Otherwise Alistair does and doesn't become king at the same time. Loghain does and doesn't join the Grey Wardens.
From a writing perspective, as a writer, characters are not necessarily defined by their sexuality unless it is important to their story. Many characters can be interchanged between straight and bi without having a dramatic effect on their personality. And if you are writing a character to be able to be either sexuality, that can easily be written to prevent contradiction if it has been planned from the start.
To put it another way, if Hawke can be written as a character and be either male or female, straight or bi - why can't the LIs be written to be either gay or straight depending on the game requirements of the player?
It doesn't make every instance of those LIs become bi. I know a lot of people who played Jade Empire who thought Sky was heterosexual because they never came across the option that had him romance the male PC.
It's kind of the same deal here. Of course the truth has yet to be revealed regarding DA2.

We have a little while still before we know for certain.
Conan