I wonder why they don't do it though considering the instances in Dragon Age (hilarious as the love scenes turn out).
Okay, if you want off topic...
Most of the gay community would not prefer a new, explicitly bi/gay character in fear of a stereotype or an over-the-top characterization. However, turning one of the existing squad members wouldn't really work either for several reasons - including the alienating of the straight players, who are still the vast majority of Bioware's customers. Just think about the freaking out and backlash if someone would find his male Shepard accidentally hitting on Jacob for example.
The other suggestion that I've seen was to offer a choice of sexual orientation at the game's start, but it'd also be unmanageable.
But in the end it was the Bioware writers who announced that their own image of the main character is not a bi/gay person. DA1's hero was much more of a blank piece of paper, but Shepard is an actual and pretty well defined character, even if it's up to the player to make the moral choices.
In the end it's not an easy question, and whatever anyone may thing, Bioware is still one of the few devs to even pay any attention to the issue...
Actually, after playing Dragon Age I was pretty disappointed with the squad interaction in ME2. Hopefully they'll take the best of both worlds for the final chapter...
Yeah, the downside of the increased team was the lack of interplay. Possible combinations explode pretty quickly and it becomes increasingly problematic just to keep track of them. In fact I'm quite amazed by all the dialogue variation with Liara in the SB mission and its aftermath, must have been a big effort to properly track and cover all options.
By the way, Bioware seems to believe that the PS3 version has sold out in many places, they've opened a topic in their forum to help people who can't get their hands on a copy. If it's true then congratulations for the well deserved success.