Firstly i'm glad that s/s romances were finally confirmed.

Even though i'm straight the more options the merrier I say and who my shep ends up with is usually based on personality anyways.
But I just don't understand all the oposition. Here's my opinion on the subject:
To people saying that changing a characters sexuality will change their personality and therefore ruin the character...
Firstly, sexuality is a complex thing, from alot of what I read before in these forums (specifically concerning who people seem to think could be bi), people seem to think that sexuality has a significant factor on the personality.
It doesn't have to. The effeminate gay and the butch lesbian are stereotypes. In fact, with this logic, Kaiden (who has stereotypical feminine attributes) and Ashley (who has stereotypical masculine attribute) would be both gay, which they aren't. So what i'm trying to say is that a character expressing interest in the opposite sex does not need a personality shift to justify it, and their sexuality itself does not impact on their personality, they are independant issues.
Secondly, for those who say X couldn't be bi since they haven't been shown to be before...
sexuality is often a subtle thing, most people don't go around with glowing signs on indicating what they are. This is expecially true for bisexuals, but if they only show interest in one gender, people tend to assume that they are straight/ gay. This is not true, they have merely not found someone of the same/ opposite sex that they are interested in.
Excuse the crappy metaphor, but a guy could only go out with girls with blue eyes. This does not mean he is not attracted not girls with other eye colours it could be because there are only blue- eyed girls in the place he lives, it could be because his culture frowns upon pairings with different eye colours, it could be because there is simply no brown-eyed girls that he likes as people. But this does not mean he wouldn't go out with girls with brown eyes, or green eyes, or even orange eyes (ok maybe not orange eyes). And then there are the people that are not sure of their sexualaties and discover who they are later in life.
So bottom line, a person who has not expressed interest in the opposite sex is not neccessarily straight.
Finally as for the alien issue... (as a for note, since I really don't know anything about how cross species romances would work in real life, because, well it's virtually non existant in our real world, and therefore what is below is based on speculation and opinion.)
Now this is a matter of an opinion, but Garrus, Tali, Thane and Liara, being aliens, would be, IMO, pansexual. That is they are interested in people, not based of who they are physically, be it human, or male/ female, but who they are emotionally. So them being interested in a same sex shepard is perfectly plausable for me. There would be no children anyway, so it would hardly be any less taboo for them than interspecies romance.
Also, as for the whole fear people seem to have with people of the same sex coming on to them, it's perfectly possible that there will be clear indicators to iniate a romance or not, and getting out of one will probably be easy as well. I like how it was done in DA and DA2, with the worst you get being a minor dissaproval loss for backing out in one case, with the rest being perfectly ok with rejecting you.
So all in all, the content is optional, perfectly avoidable, and not at all gamebreaking, so peoples fears of the game being ruined are really unfounded. I'm personally for the all Bi option, and I wouldn't think it would change my view of the characters at all if handled well (for example, I like Garrus as a pruely platonic friend, but if he s/s romance was intiated only though you relentlessly persuing him, and therefore unoticeable to those who didn't flirt with him, I think a s/s romance with him would work, and not effect others who didn't wish to persue it.) Really there are much bigger things to worry about in this game, and i'm just relieved that Bioware is finally apparently making this step.