Bioware is allowed to make that decision though. In DA:2 they went with the 'everyone is bi/playersexual' approach. In DA:I, they went the exclusive route. If people dislike the exclusives route, then I think people should absolutely make it known that that approach is important to them. Both sides have their merits! But for DA:I, that isn't the approach they took.
Having dialogue in the game does not mean they were hiding a possible bi romance path for X character. This type of dual dialogues has existed in numerous bioware games in the past. If this just cropped up now, I would be of a different opinion. But it isn't new. This has been in almost every bioware game.
The reason that they went the route of choosing multiple sexualities is that they consider representation more important than making everyone available to everyone. And, as a matter of fact, they said that if they had made all options available to all people, there would have been far fewer romance options anyway--probably four, the same as we got in DA2.
I respect their decision, and I respect that they chose who they did to be straight, gay, bisexual, and pansexual. It's okay that they just are the way they are because Bioware said so, even if BioWare changed their minds about how they were going to be.
Personally my reason for wondering whether there's more reason behind Solas being straight than just "because he is" is more to do with his overall lesser amount of content. But he's probably just straight because he is, and much as I want more content, it is probably not something we're going to get. I hate to say it, but the more distance I get from the game, the more I feel like the end to Solas's romance was indeed final. Romantic closure will probably be denied to us in favor of more plot-related and area-related stories.