I'm still not seeing from this what sort of romance story a homosexual LI could have that a bisexual couldn't have. I'm not at all fond of the "troubles of being gay" route, but it could be told just as easy with a bisexual character, if they feel they absolutely have to do it.
They could have companions and NPCs that are gay without making them LIs. Celene is a lesbian.
I think that there is difference in what comes to stories depending on your sexuality. For example lets assume that you are a son in a noble family and you happen to be gay. You are expected to to find a girl and marry her. Because you are gay then I assume this would be quite agonizing thing to do because you would like to marry a man. If you happened to be a bisexual man then this might be also agonizing to you but because you are bisexual then there are options available to you even if you can't choose any man. I don't see that this situation is as agonizing to bisexual than it is a to a gay man I would say that the tone of the story is quite differrent depending on your sexual orientation.
Another example let's assume that you are straight guy who has lived all of his life in an isloated island where almost everyone is a man. The only available women are too old or are married with somebody. So you can't have a romantic relationship with anyone. But if you happened to be bisexual then there is a change that you would find a partner among the men but you are not bisexual.
So why these stories should have anything to do with a romance? Well if you are that straight guy then it might be a big deal to you that you have been living in that island and not have any romantic relationships and you might not want to discuss about that not even with your closest friends. So that guy could only tell that story to an LI and I think that trust which he will give that LI makes the relationship more deep and meaningful.