In fact, this is not the case and the "parts" that you are talking about are present in game.
For example, Gale and Wyll had romantic relationships with women in the past. Gale also pays attention to Shadow when they first meet. These are a kind of "flags", both of these characters so far look more like straight man. Shadow also looks like a straight girl. Why? Because in her "romantic dream" (if she's not lying) there was a man. These "flags" indicate character tastes. Writers prescribe these details (but not very clearly), developers leave choice to players, how they want to perceive it and what they will do it at all.
Yes, I think it's a bit strange if Gale approves of an affair with a male character. But you know what? The other player may see it differently. And in the end, he may fall in love with this particular character. He doesn't need another, but a well written gay character, he wants only this one. Because of the personality. And then what to do? Play as a female character? But this is a role-playing game, with choices, where you most often want to create something that you want for role-playing, and not only "for romance with a character", but you also want this romance.
That's why it's easier to "make everyone bi", let everyone decide for themselves "who is who", flags with characters personalities do not go anywhere. If someone "breaks" atmosphere by ignoring these "flags", then this is their own business.
Of course, this is only EA and my own impressions of the characters. But I am sure that there will be more of these "flags", and you will be able to better understand for whom this romance was written more, if, of course, you want.
Only half of what @Some_Twerp753 is talking about is in the game. Gale and Wyll do discuss their romantic relationships with women and Gale even rebuffs Astarion. But there is no mention of any sexuality when the player pursues romance (or, more likely, when Gale and Wyll pursue romance with the PC). There is no Cassandra moment, when the NPC acts to reinforce earlier dialogue/backstory. The companions can all be bi, sure, but some type of "I've never been with a man before" dialogue, acting nervous, and/or not actively pursuing male PCs would be more consistent with their (stated) history and personality flags.
