I think Bioware was just doing what was best for people in the long run to have their own personal best experience. A lot of people like putting themseleves in the game myself included. I know people think its unrealistic to have all four romances be bi, but to be honest it SOOOOOOOOOO hasnt been confirmed by any developers. A statement I will only retract when a developer actually says otherwise. and Also, if they have done it, then think of it this way: The characters are amazing. Anders/justice storyline, Fenris being on the run and a superwweapon, Merril and any storylines relating to her dalish heritage (know some of it but may be spoliers and unconfirmed) and Isablea who is just amazing no matter what, and we see more depth to her character in this game( she actually makes some pretty morally solid choices for someone in such a dishonest line of work),. My point? The characters were created to be defined by their opinions and your relationships with them on a platonic level, not their sexual identities.
Romance is simply having your character take a more romantic interest in them as oppossed to a platonic interest.If the developers made Hawke's many LI's all bi, it is because they want a personal experience for every person who plays it. That said I agree that as it is OPTIONAL content, that flirting should be the only way to initiate it, and then from that point have the dialogue unlocked for that. A Straight or Gay person should not have to listen to unwanted come ons or have the "hope you dont mind I like [insert gender] too" talk, from any of the LI's.
On the flip side if they havent made all romances BI i'll be fine too because certain characters did seem to have pre-defined sexual identities before DA2. But this is by no means a reason they shouldnt add another layer by making them bi, as long as the "hope you dont mind" never appears.