Retcon? Show me in-game documentation that explicitly shows that they are straight. You'll not find any.
You assuming that the character is straight is what's being retconned. Keep it to yourself.
Bisexuality implies that a person is attracted to both genders. Just because a guy is in a same-sex relationship doesn't mean he doesn't comment on some girl he finds attractive.
Would it really "fit" for Anders to have said "I just want a pretty partner and the right to shoot lightning at fools"? It just doesn't work. It's far simpler to just stick in "girls", especially since Anders was not initially going to be in DA2 and he was never going to be romanceable. What was the point of using gender-neutral descriptions?
Then there's Kaidan. Ignoring the fact that the same-sex scenario was cut out due to anti-gay pressure at the time of release, Kaidan never explicitly reveals his sexual orientation to the player. The hidden sexual tension between them is pretty obvious, only to be escalated in DA2 when he tells you that losing you was like losing a limb. There was no "retcon".
They don't need to explain why they were always bisexual because ACCORDING TO THE WRITERS they were always bisexual.
That's easy.
-Kaidan is only attracted to female Shepards in ME1. Never shows any interest beyond friendship in male Shepards or other males in general.
-Anders consistently flirts with female party members, and female Wardens in DA:Awakening. Female Wardens have options to lightly flirt with him as well, which male Wardens do not. He never shows interest in any other males, party members or otherwise. He also reacts with disgust when Oghren makes a joke about him having sex with a male Templar, and it's not just because it was a Templar, since he shows attraction to the female Templar that always recaptures him.
I suppose it is a matter of opinion and I've already said it, but to me it makes no sense that Kaidan would be attracted to female Shepard but not male Shepard in ME1, and then both in ME3. It's the SAME Shepard. I would have been all for it if they decided to make Kaidan bi from the get go in ME1, but they decided not to go that route, and I believe once that is decided, they should stick with it. If the purpose for not making Kaidan gay in ME1 was to offer a natural build up for the relationship, then why not withhold him from female Shepards too?
With Anders, if they had planned to make him bi from the beginning, it would have been pretty easy to alter his character a bit in Awakening to portray that. But they didn't, everything he says and does points towards him being straight in that game. And I highly doubt merging with a spirit would make is sexual preferences change.
The writers will always say that they were "always bisexual", they will never admit to retconning things and saying that they weren't would devalue the people that choose to romance them with male characters now (since it was already put in). It was pretty clear cut that these guys were straight in previous titles, it was just changed to both appease the audience (men who wanted to romance Kaidan in ME1) and fit the game better (everyone is bi in DA2!). You will never convince me that Anders in general (not just his sexuality) didn't undergo some huge retconning to fit DA2.
Just because a straight person doesn't go around announcing they are straight at every turn doesn't mean they are bisexual. People never "come out" to being straight. It just doesn't happen. Pretty much all of the bisexual and gay characters confess to their bi- or ******-sexuality at some point in the game (Leliana, Zevran, Sera, Dorian, etc.). This is not he same for straight characters, Alistair, Morrigan, Sebastion, etc. never explicitly say "I am straight!", it's just implied in their actions.
And I think, with Cullen, it has also always been implied that he is straight. He has only ever been attracted to females. If he was intended to be bi all along, then they would have had him crush on a male Amell/Surana too. If this was the case in DA:Origins, then I would have been completely fine with him being bi in DA:I. But it's not. He only ever liked females.
As far as his rejection for a male Quizzy goes... his rejection to a male quizzy and a female Qunari/Dwarf quizzy is different. To female Qunari/Dwarves, he stumbles around a bit and basically says "He's too busy and wouldn't have time for them". To males, he tells them something along the lines of "he would be unable to return their affection" pretty plainly.
To me, I take his line to males as a very nice way (Cullen is a nice guy after all) of telling them he doesn't swing that way. It seems like a way of saying that he would never be attracted to them, even if he tried. His rejection line to female Qunari/Dwarves seems like a typical way of turning down someone of the gender you are attracted to, but don't want to be with (I.e "I just don't have time for a relationship right now"). Cassandra, Sera, etc. are far more bold and blunt with their actions, so it seems more suiting that they would be more blunt in saying "i'm not attracted to your gender" than Cullen would.
This is not a matter of only wanting characters to be straight, it's a matter of wanting characters to be consistent across games. I would feel the same if they decided to make Isabela a straight only option in DA2, or if they brought Sera back in a later title and made her bisexual.
Maybe from now on, every single NPC should be have it written in huge letters above their head what their sexuality is, because it seems like that's what people here need.