Characters don't say they like both characters. From a headcanon perspective, you can think "Oh, straight/gay/lesbian" when you first see them. When they express interest in one someone of one gender, you can think "Oh, they're interested in people of that gender." However, when it's revealed (either by the characters themselves in-game or the developer testimony or what have you) that they are in fact Y instead of X, it's highly stupid (not to mention egoistical) to say "Well, the character in this playthrough didn't tell my character in said playthrough to his/her face, therefore I choose to discard the lore and believe what I want to believe based on what I see on the screen."
Except that's really not what I'm saying. Not in the slightest.
To reiterate my position, if there is no clear indication that a character is bisexual, then I am probably free to think that they are, or that they aren't. Either way, it would be my own personal interpretation (that is to say, what I personally accept and what makes sense to me.) Notice that in no way am I saying that you are right or wrong by calling them bisexual; I am simply saying that you don't really know--and as such, it is up to you to interpret it however you want as the writers chose not to give you a definite answer.
When I said they aren't technically bisexual in a previous comment, I was really saying we have no real proof that they are. The fact that the game gives me the option to romance these characters regardless of my own character's gender is irrelevant, and most certainly is NOT an indication of the lore telling me that they are bisexual. Just because I can romance them with the same sex does not mean they are, in fact, bisexual. Rather, it means I have the option to try a romance with them. That's literally all it is. It is NOT proof that they are bisexual as is the case with Anders and Isabela.
Now, I'm going to admit that this could change and it might one day be revealed that Fenris really is bisexual. If that happens, I'm sure nobody would be pissed and delete their DA2 saves because OMG RETCON, since you can't possibly retcon something that was never even stated in the first place.
Moreover, David Gaider did, as a matter of fact, mention that he does not consider the DA2 romances bisexual for this very reason. I would dig out his post for you but it is buried somewhere beneath the depths of the legacy forums. Do I take his statement as gospel proof that these characters aren't? No, certainly not. But it's something to ponder.
Personally, until I see in-game indications (or similar), I'm sticking to the idea that they are equally likely to be of whatever sexual orientation there is. Thus, it's convenient to headcanon and say Fenris or Merill is gay in my playthrough. Does not mean I'm not open to further revelations regarding their actual sexual orientation.
I guess it's safe to say that I identify more strongly with the idea "because they don't say it, doesn't mean they aren't" than the idea "because it's not made obvious, I am free to assume what I want."
That is fair enough to me, because for all intents and purposes, we are arguing over something that we don't know the real answer to (unless your super secret internal sources have shown you something we don't know about, Allan.
)