I much prefer the slightly credulity-straining conceit that we just happen to have met a bunch of bisexual people (which could happen, there are presumably at least thousands of them in Faerun, if not more), then the less ideal situation of people being disappointed that they cannot romance their favorite character while playing as their preferred gender.
I prefer having all companions able to romance a PC of any sex. For a simple reason : player freedom (described above).
But that doesn't make the companions bisexual in my eyes. Because multiverse.
In my mind, every time I start the game with a new PC, a new instance of the fictional world is created. In this instance, all companions happen to be interested in (at least) people of my PC's sex.
For instance, if I rolled a female PC, Gale is interested in females. If I rolled a male PC, Gale is interested in males. Unless the story explicitly portrays the NPC as bisexual, there is no need to assume that they are (they may still be, it's just not mentioned).
As a player I may play the game with PCs of various sexes, but each PC will see only one world, and this world will be consistent.
