This is the reason I don't understand the sexuality argument (and I'm just speaking for myself here.

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I'm one of the lucky few whose sexuality doesn't
fully define who they are as a person. It doesn't define my personality, it doesn't really define any of my likes or dislikes, it doesn't define how my family or friends treat me, it doesn't define what foods I eat or what animals I prefer as pets, what places I like to vacation, what social circles I run in, etc... In fact, the only thing I think it DOES define is maybe some of my political beliefs, and of course, who I'm attracted to.
In Thedas, however, I don't see one's sexuality influencing their world/political views in anyway,
because it's not Earth. The majority of people in Thedas are tolerant of s/s relationships. Other things influence those views, like whether you're a mage or mundane, rich or poor, Orlesian or Ferelden, Andrastian or barbarian, elf, human or dwarf. These things seem to define one's 'character' (in regards to Thedas) more than their sexuality, because unless you plan on settling down and having a family, it's pretty much a non-issue.
That being said, characters can have preferences beyond sexuality, but at the moment, sexuality is the only recognized preference. I imagine Dragon Age: Origins wouldn't have been much fun if Alistair was only attracted to redheads, or if Morrigan refused to start a romance with anyone who wasn't a dwarf. Admittedly, sexuality is a preference that holds more water than what color hair your character has, but this is more or less just to prove a point.
If you wanted to romance Alistair, but could only do it with a redhead, it would limit what you could do with that character. Players who are limited by sexuality are in the same boat, only, instead of having to change their hair color to red, they have to change their gender.
Now, when I personally ask for all bi LI's, this is what I mean (once again, I'm only speaking for myself) that the sexuality of the character be subjective to the player. Basically, if you want a straight Alistair, Alistair in your game is straight. If I want a gay/bi Alistair, I can hit on him with a male Warden, and then wow! I learn he has an interest in men.
I'm not asking for all 7-8 characters in your party to be bi, or even for everyone in Thedas to be bi, just that the 4 LI's we actually get be accessible to everyone, regardless of gender. Some people think this is unrealistic, and I don't think they're wrong, per se, I just disagree?
Looking at DA:O, we had an ex-templar knight, a witch of the wilds, an Orlesian bard, an Antivan assassin, a Qunari, a dwarven warrior, and a circle mage. These people weren't all pulled from the same isolated corner of Ferelden. They came from all over the place (some from different countries) from different backgrounds and societies. If there were ever a group to have 4 LI who
could swing both ways (I say 'could' because if you wanted them to be straight in your game, they could still straight), this would be it.
Then I look at DA:2. Excluding siblings, we've got a widowed templar's wife from Ferelden, an apostate mage, a runaway elven slave from Tevinter, a dwarven merchant prince, a swashbuckling pirate, and a Dalish Keeper's First. Once again, these characters don't all come from the same isolated corner of Kirkwall. They come from different backgrounds, counties, and ways of life. If there were ever a group to have 4 LI who could swing both ways (etc, etc...

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And that pretty much sums up my stance, I guess.
I think the main problem that I have with people putting so much emphasis on the importance of sexuality is that, when looking at the current line-up of LI's, I'm not judging them based off their sexuality. I'm instead looking at their appearance, their personalities, their back stories, etc, because to me, these are the elements that make these characters who they are. The
only reason I'm curious about sexuality at all is because I don't want to set all of my hopes and dreams and fan gushing up on
one LI, only to have them squashed and come crumbling down around me when I'm told, "Or, sorry, that character you thought was completely fascinating? Straight. But there's this other guy/girl!"
Because while I may grow to like that other guy/girl (heck, I'm so easy to please in the character personality department that I'll probably actually
adore them in the end) I know that when I pick up the game, I won't be chasing them because of their sparkling personality/intriguing history. I'll be pursuing them because a spotlight has been shined on their sexuality, notifying me to the fact that they're the only available s/s companion for me.
Anyway, I do think BioWare could effectively write all bi LI's if they had the means/desire, and if they don't have the means/desire, I certainly understand.
I do appreciate what we are getting in terms of bi LI's, but as the saying goes, 'want mooooar.'

Plus, I can chalk up half my feelings to pre-release excitement/fears/hopes and dreams, so who knows if I'll even feel the same way after March 8th?