I think we're just going to have to agree to disagree
This has been a nice conversation... though you do like to exercise that condescending tone lol
None of the characters do whatsoever, as it was clearly determined what they would be based on player choice. At best, they are asexual until the player tells them otherwise, which isn't realistic in the slightest.
Well, they are characters. They don't have identities; they are created, and the player to some degree "creates" them as she plays (since this is an RPG; the player is a sort of player-writer). She also creates the narrative (within a strict set of rules, obviously; her options are not limitless). But no, as soon as Fenris is in a relationship with my Hawke, he is either gay or bisexual. I don't see how that makes him less realistic. If I decided to replay the game and play a female Hawke instead, then tried to hit on him, but the option was no longer there... I guess that would make him gay. But tell me, how would that have made him better? The only difference is that now I can't romance him with a woman. How is that more realistic or compelling? His romance with my male Hawke hasn't changed at all.
You have just hit on the real target of the issue. Companions were "player-sexual" as you call them. This was something David Gaider addressed directly at the panel and it's also a phrase he vehemently dislikes. For one, companions that are player-sexual aren't realistic or believable, because their orientation is based on your choices. How can that be authentic in the slightest? Secondly, it more or less suggests companions do not have the self-awareness or appreciation to choose for themselves. You have to tell them what they are. This essentially leaves them being nothing more than computer programs where you are inputting data and telling them what they are based on your choices. If you haven't figured out why that isn't authentic or believable yet, I'm not really sure what will.
You keep mentioning David Gaider's panel; are you talking about this one? I've listened to it several times, but I admit I don't have it memorized (lol). When does he say the DA2 characters are not bisexual, and when does he talk about player-sexuality? Or are you referencing a different video?
Again, I don't understand how a companion who is "player-sexual" is not believable. He has sexuality. His sexuality is whatever I want it to be. I'm essentially "writing" him. When I sit down to write, the characters I create are plenty believable. I may make them gay, straight, or bi. That's just how the process works. When I turn DA2 on, I am about to "write" my next adventure in Kirkwall. In this particular adventure, I will romance Fenris. In my previous adventure, I romanced Isabela. Both were well-written and emotionally fulfilling romances. And...?
Companions don't have self-awareness or choice -- they are characters!! FYI: They are computer programs, lol! Companions in video games are not real people... because we don't have multiple playthroughs in real life, nor can we reload in real life. Characters are characters. A narrative is an illusion... we purposefully trick ourselves and emotions into thinking it is real, but it's not. But so good game developers try to create worlds and characters that make it easier for us to trick ourselves and enjoy ourselves as we romp through Thedas. Not being able to flirt with Cullen doesn't really make the world more real for me. It doesn't make him more believable than Fenris, either. I'm going to assume Cullen will be as well-written as Fenris. I still don't see how one is more real than the other. Both are illusions. One being available to me and one not being available isn't going to help with my immersion any better.
Much like companions will differ in their view in regards to the world, politics, tastes, their sexual orientations should also play a role in who they are as individuals as well. Does this make sense? You should not have absolute control over what orientation a companion is. That is not realistic.
It would not be realistic in the real world, but a game is not the real world. I am starting to see what you mean by "realistic." I disagree that I shouldn't have control over a love interest's orientation in an RPG. The nature of an RPG, in my opinion, is for me to play in a user-determined universe. I don't see a problem with getting to pick who I romance. The only thing that would frustrate me is if the romance were poorly written. So long as the romance is well-written, and I am moved and excited by the romance, then I think it's a job well done. I got what I payed for.
If you cannot recognize the obvious importance of one's sexuality and how it defines you as a person, I'm not really sure there is much convincing that can be done. The fact that you compare someone's orientation to being on par with one's eye color or hair color more or less says it all. In my personal opinion, one's sexual orientation has a MASSIVE impact on how one thinks, acts, looks at others, and perceives their environment and society. It influences and encompasses his/her entire being, and isn't merely a set of options one chooses from. That, I feel, misses the point of sexual identity altogether. This is likely why you don't understand why having a sexual identity is so important. You fail the recognize the significance of it.
Oh yes, I think our thinking differs greatly on this matter, though I won't explain it in such a condescending way as you did
No, I do not think sexuality defines you as a person. I don't think my sexuality affects how I think, act, or perceive the environment around me. It certainly does not influence and encompass my entire being, goodness gracious!! Sexuality may be part of one's identity, but identity is fluid. No, I'm not saying sexuality is fluid (although for some it may be). But identity is meaningless. There really is no such thing as a core identity. I may fail to recognize the significance of sexual identity to you -- but I do not fail to recognize its significance (or insignificance) to me.
For me, my orientation affects what I find attractive in a person, but that's all. Of course I am also agender/gender fluid so I may be bringing that perspective to the table. 
So anyway, yes. When I'm writing a character, his or her sexual orientation is way down the list of things I consider important and fundamental to his or her personality and world outlook. In fact, I frequently change my characters' sexualities midway through a piece, or sometimes from story to story. Generally, it's still basically the same character.
Having a sexual orientation, as you said with Dorian, adds to his story, history, and cultural makeup. It's another layer of the complexities that make up being human. BioWare games generally have stayed towards social norms having the typical straight romances and on occasion a gay romance every now and then. They are now taking this more seriously and not just merely writing a personality and some lines for these companions, but rather trying to bring them to life, and having a sexual identity goes a long way to furthering that goal.
I agree that Dorian's sexuality is clearly going to add something to his story. By the way, BioWare have only had two gay romances previous to DAI, so I think that's a little less than "every now and then." That is the only reason I approve of the 2/2/2 model: because gaymers sorely need to see gay characters in their games and have romances that have been created EXCLUSIVELY for them. Not because a playersexual Dorian or Sera would be a lesser character -- they wouldn't -- but because it will make gaymers happy and feel as if the industry cares about them (and doesn't relegate them to a gay planet, hint hint, SW:TOR...).
But that's more a problem with the outside world, the real world, than the world within the game. If the real world weren't so homophobic, we wouldn't have such a need for gay and lesbian romances. Playersexuality would be perfectly fine if the real world were 100% inclusive of all relationships.
You merely see a lack of choice as a downside to you from a player standpoint, but think about how the writing for the character is compromised and made neutral in order to cater orientation to one side or the other?
I don't believe the writing for Fenris, Anders, Merrill, and Isabela was compromised and made neutral, sorry. I think they are marvelous characters, and I found their romance every bit as fulfilling as Zevran's (only DAO romance I played) or Kaidan's (only ME romance I played). People use the word "cater" when they want to shame a certain player base. I certainly wouldn't call playersexuality "catering." I would simply consider it more cognizant and considerate of player choice. But hey, even if the team wants to distance themselves from the concept of playersexuality, on account of bi-erasure, then I'm 100% behind them making all romances bisexual (or pansexual). Have the love interests showing interest in both genders when they are not being romanced, and there you go. No bi-erasure, and 100% player choice. So long as we have well-written characters, we're good to go.
It ultimately comes down to a matter of opinion, but it's irrefutable that the approach the writing team is taking is more realistic, albeit more restrictive. Everybody isn't bisexual. People have strong opinions and feelings about who they are and how they view others. All BioWare is doing is addressing these real factors into their characters now more than ever. Dorian will be homosexual and that will play a role in regards to how he acts and who he is as a person. If he was just "player-sexual," he would essentially be an empty carcass until you tell him what to be. If you still can't distinguish the differences at this point, I believe we'll just have to agree to disagree. BioWare made it clear "player-sexuality" will not be happening again after DA2, so it really isn't much of an issue at this point.
OMG Your wording is just so... lol Hmm. Yes, we'll agree to disagree. But I'll do so without insulting you
Now I'm going to go read some fanfic about my poor empty carcass boyfriend Fenris. Ta~ta!