They don't need a reason to make someone gay. They can just be gay. But with an already established character it can be a bit strange for this to come out of left field. Then again, Anders had no reason to mention his sexuality to the Warden, so he could very well have been BI. I dunno, I'm probably wrong on this whole thing.
Anders has confused a lot of people because I think he was implemented in a way that creates dissonance because it's not how we expect a bisexual character to act. We've been conditioned by media to expect a bisexual character to explicitly state in no uncertain terms that they are bisexual. Otherwise, people just assume they are straight or gay. With Anders, because his "flirting" with Nathaniel and teasing of Oghren was really vague, many people just assumed that he's "default straight" because there was no explicit evidence otherwise. Then, in DA 2, he "comes out" as bisexual and people have to readjust their schema. They had put Anders in the "straight" pile and now they have to move him. So people want an explanation as to why they have to do this.
I'm not blaming the people who think this way. It's how we've been conditioned, so it's natural (well, not natural, but understandable?). However, it's the people who continue to argue that he was never bisexual at all because the game didn't provide concrete proof, even after being told that this isn't really how it works; these people I kind of blame. Not that I'm saying that you are one of those people, because you are clearly not.
If anything, I find Anders' situation fascinating because it's what many of us want for our characters: to have their sexuality just be another characteristic that's not given this tremendous weight. But when it does happen in a way that's fairly realistic, people have issues with it. I think it's an interesting thing to watch.