While I can see the argument of "Just make the non-LI companions be the LI's" I don't think it's anywhere near as simple as that.
Let's look at the companions in Origins. Say they had the 4 bisexual approach that so many want but also want to represent all sexualities. Leliana, Morrigan, Alistair and Zevran are all bisexual, so we'd need the other companions to represent the different sexualities.
That gives us Oghren, Wynne, Sten shale and Dog. Erm...
And that's the problem. With practically half of the companions being LI's saying that they all must be bisexual is really limiting if you want to include that diversity. If they wanted to do it in Origins at least one of the above would have to be scrapped. If you want to represent everyone then out of those four or so non-LI's you'd have to have one straight and one gay at least, so half of their sexualities are predetermined before the characters even have a chance to be conceived.
What about a bisexual companion who isn't an LI? Not likely to happen if approximately 50% of your cast have to be bisexual by mandate to fill the LI quota.