Oh, i think we should give Bioware more credit, Saibh.

In Origins, Leliana was bi but seemed to be more default-hetero; a female Warden had to choose dialogue progressions more selectively to get Leliana to enter into a romance. Similarly, Zevran seems to be an equal-opportunity playboy, but it is only a small part of his sexual identity: he is more than anything a product of his fortune at a young age, being born an elf, an Antivan and of course his training as a Crow.
I'd wager that the all-bi decision in DA2 has less to do with general sexuality and more to do with the fact that we gamers play as Hawke, a fixed persona, but a persona we each get to shape in our own way. It's about us, about our Hawkes. We become (co)authors of his or her story, so who's to say one person's female Hawke is into Isabela and another player's Male Hawke is also into Isabela -- not to mention any single gamer's multiple play-throughs of the game as different Hawkes.
My theory is that each companion has been programmed to have a valid explanation and unique point of view about whichever Hawke is romancing them. It's not so much that, say, Merrill is a bi Dalish elf in
all stories, but that the developers have made it so that she is
available to
every version of Hawke, you know?
Or maybe I'm giving Bioware too MUCH credit!

Funny FYI: When playing a female and romancing Leliana, I found that one line about sharing a love of shoes with her a positive bonding moment towards a lesbian romance. But when I tried the same dialogue options as a male Warden, she defaulted to friend. We were like Will and Grace, apparently.