Some of my own views as a player...
From my perspective, I consider NPC's as PC centric in regards to romance: Meaning that that are none of the above unless otherwise confirmed.
In regards to Merrill, for example, and based on her farewell in DAO (if you played a Dalish Elf) suggests she and your character had a close relationship. It could be a close friendship, or it could be because she and your character could have been romantically involved (no matter gender). There is nothing there to suggest either way, other than they were close: there is no distinction either way.
What I dislike about these kinds of distinctions that are often made by some that is based the metadata (knowing that a particular NPC can be romanced by either gender) is is that it is often used to describe a NPC as Bi-Sexual as opposed to a PC centric observation. If you play through the game as a female PC (without using metadata), and romance Merrill, as an example, your current female character has no knowledge as to whether or not she is straight or bi-sexual, although as a player you may know that she can be romanced by either gender: that DOES NOT, make her bi-sexual.
Lelliana is another example. If you play a male character, she states that she is bi-sexual, but that does not make it true for a female PC.
Morrigan is yet another example. If the PC is a male character, she warms up to your PC quite readily. The ONLY confirmation one would have that she is straight, however, relies on metadata learned from playing both genders on separate plays through the game (or from reading the forums).
To simply state that an undeclared NPC is bi-sexual, gay, lesbian, etc. is a falsehood: the available romances.
So, what I ask for in this discussion, is for people not to place labels on a given NPC character regarding their sexuality based on metadata: as always, romances in BioWare games have always been PC centric rather than some perceived lifestyle based based on knowledge your PC does not have.
In other words: The PC's perspective prevails over metadata.