After trying in vain to articulate why it is I don't see the problem with Bioware's choices, I think I've found the answer. It's because I don't, in any way, associate a character's sexuality with that character's personality.
This is what I see as the nasty undercurrent in these types of discussions. It often boils down to players of one orientation not wanting to think that their favorite NPC would be of a different one because it changes their interpretation of that NPC ('oh he didn't seem
gay, where was the lisp?' or 'but if she's bi then that means she's a slut and I don't want my character romancing someone like that!'). Okay, maybe it isn't quite that bad, but the problem here is that many people seem to think that a character's personality should somehow change based on their orientation. There seems to be this notion that it is 'lazy' for a writer to write a gay character and a straight character the same way. Which smacks of stereotyping and is more than a little absurd and offensive. Made moreso by the fact that many people seem invested in breaking the fourth wall just to whine about this (with the bonus points of fantasy and sci-fi worlds to which we are importing modern sexual stereotyping).
All the above said, I will say that I think the notion of the main-character centric romance is more of a problem in the Dragon Age series than in the Mass Effect series. But that is just because romance is handled so poorly. At least in Origins, I haven't played the rest.
In DA:O it is possible to just hang out around camp, flirt, and screw. It becomes this whole little side thing that plays out in your time-stopped camp while the world falls apart around you. Because of that I suppose you could say that DA:O has more of a (very rudimentary and primitive) dating sim-ish aspect with the cute flirting and gift giving. In that context Sabby's complaint (if that really is the whole of his problem) seems more appropriate. Maybe you want to be able to play around, be shot down, and navigate the orientation politics of your little group. Or maybe, like me, you want them to stop goofing around like horny highschoolers and get out and save the world (the only responsible time to goof around like horny highschoolers while
saving the world being, of course, the latter games of the Persona series
So lets compare that to the first Mass Effect. ME is distinctively and intentionally cinematic. Commander Shepherd is essentially the current generation's John Carter or James T. Kirk. You are a big, damn space hero in the midst of saving the galaxy. It should just be part of the tropes involved that if you want to, you are going to get the girl, boy, or monosexed blue skinned alien. Moreover, you can't just talk to them/bribe them enough so that they will give you a sex button to push any time you are in camp (no matter how much I liked Alistair's sex button, that seemed far more like ridiculous pandering than anything to do with orientation). Sex in ME has the possibility of occurring at only one point in the story: at a tense, dramatically appropriate time Shepherd's romantic partner will approach him/her. It serves as a strong conclusion to the romantic sub-plot and really adds some emotion and humanity to Shepherd's arc. In that sense I feel that it is not only convenient for the writers, but actively good for the story if you have as much freedom in possible in choosing Shepherd's partner. Who they become intimate with is a telling thing about your particular iteration of the character and it would be foolish to let orientation concerns stand in the way of what is ultimately and openly a story that is incredibly individual and subjective to every player. (Not to mention the issue that as a gamer I am not going to go back and play through ME 1 again with a male Shepherd that I don't want to play just because the developers decided that my favorite character wasn't into girls).