I know that's incredibly difficult considering how far in advance we get information on the companions...and that's why I hold the secret belief that in an ideal world BioWare wouldn't reveal the companions AT ALL before launch.
Heh, could be fun. And while I'm not sure if going that far is feasible, I would absolutely love it if they didn't reveal who is romancable. Let people discover it in game. Going into the game without knowing who is a potential love interest, and having to find things out for themselves by trying things out. I find myself attracted to one of the characters. I try to chat her up. And she shoots me down because quite simply she's not interested. And the one that would be interested is has already hooked up with someone else because I spent too long chasing the uncatchable. As disappointing as it would be, that seems so much more...real...than knowing that, if I pick the right dialogue options, Character X will fall in love with me.
And having restrictions on who NPC will fall in love with just adds to this IMO. While I do like in game romances, and yes, it does disappoint me when a character I like is not an option, at the same time, it makes the world that much more believable. Different people have different preferences. Whether its sex, race, personality, or even trivial details like hair colour. I mean, seriously, I know people in real life who don't find red hair in any way attractive. And they would have no interest in dating a readhead (crazy, I know). Can't ever see it happening in game, but in some ways I'd love a character who would reject the PC based on something like that. It would make them feel far more like a real person than the usual "will sleep with PC regardless".