Even if you go by the figures of 7.1% of people being gay or bi (which are high), the chances of all 4 romanceable characters of the same gender as the PC being gay or bi are about .0025%.
Presumably the idea behind it was that the player can choose whoever they want to be involved with and sort of ignore the rest - in other words, if you're a man and choose Gale, then the implication would be that Gale is gay/bi and that the rest aren't, and so on.
But then they have everyone get jealous of everyone else, and the NPCs are typically the instigators rather than the player, so it backfires and makes your entire party look like mirror images of the desperate sweat-soaked orgiastic chronic-masturbator Belgians who wrote them. And, like you say, it does just start to look very improbable in way that makes it all feel surreal and the characters feel cheap, to an extent that can't be handwaved away with "well, it's fantasy, it's a silly wish fulfilment fantasy".
What's interesting is that a lot of people, from all different demographics and from all different corners of the internet, aren't enjoying this angle of the game. Notably, even many people who generally really like this kind of thing are disappointed. My friend who's a big Dragon Age fan (she's even written fanfic of it which was
genuinely well written and therefore a tragic waste of literary talent) is ragging on the game for this, on the grounds that none of the companions feel like they have any kind of distinct personalities in terms of their romantic preferences, and their attraction to you feels totally baseless and impersonal given that they'd be attracted to
anyone in your place.
Larian shot themselves in the foot a bit with that because I bet if, say, Astarion had been exclusively gay and Karlach had been exclusively lesbian, both characters would have been adored by those respective communities. Gale could be a creepy bisexual loser to cater to that crowd too, if needed.