For the record, the companions in DA2 weren't playersexual. Carver was straight. Bethany was straight, Aveline was straight. Varric was straight. Sebastian was straight. The other romanceable companions were bisexual. I really didn't have a problem with that, but I am bisexual myself. I can understand how that would be really frustrating to monosexual gay or lesbian players who would like to be have more representation. HOWEVER monosexuality in itself is not what makes a character believable. All of the companions in DA2 were completely believable as bisexual people and as characters. I think DA2 was actually a very successful implementation of set sexualities - there just weren't as many sexualities as players might have expected. Those that were represented, however, were represented in a comprehensive and nuanced way.
I don't feel like DAI was some sort of great leap forward, though it's fantastic to have gay and lesbian representation. Dorian and the Iron Bull were the only ones whose sexuality felt really integral to who they are and how their romance path plays out. Maybe Solas as well, in that his fascination with a Dalish PC rings true, and the particular relationship he has with her does seem more stereotypically heterosexual. Sera's sexuality is far from the most important part of who she is, which is great, except that a lot of players hate who she is and can kick her out of the party at any point. Josie's arc makes more sense for a straight man. Cassandra, Blackwall, and Cullen don't feel like their straightness adds anything to the story.
And like syllogi said, f/f romances are the ones that tend to get shafted, and "main" romances tend to be straight. I'd like to see that change in the next game. If that means the main romances are bi, awesome. If they're gay, even better! We've never had a gay main romance.
I'd also like to see more non-romanceable characters who aren't straight. DAI is a step in the right direction with side characters like Celene, Briala, that scout in the Hinterlands, and War Table cameos from Herren and Wade. It would be nice to have a companion who isn't into the main character, but also isn't straight.
Ugh...1 of my biggest annoyances about people saying Merrill didn't make sense as being bisexual was that she was sexually naive & innocent about relationships aka "the girl next door" so it was somehow wrong, but Isabela being a walking stereotype was perfectly believable since thats how "real" bi people act 
Actually... how do you know about the Hawke twins? Carver and Bethany are your siblings, surely you didn't flirt with them? Also, you never get the chance to talk about this stuff with one of them and the other is pretty much gone after chapter one.
Aveline and Varric were never romanceable (is that even a word?) and Sebastian was the tacked on DLC prude who dreamed of a chaste marriage.
Isabella being bi makes sense. Yes, she was a bit of a stereotype but still believeable. I liked her a lot. So did my female Hawke. ^^
Fenris ... I don't know. I never romanced him. He's just not my type. I don't like broody pretty guys.
Anders... well, perhaps if he had some anger management sessions, we could have talked about it. Him being bi surprised me at first but then I didn't care about him all that much.
Merrill... soooo adorable. Yes, she was totally naive and innocent (except for the blood magic stuff). That doesn't mean she didn't have desires, she just didn't act on them before meeting Hawke. Merrill being bi makes at least as much sense as Isabella.
Individually, their bisexuality makes sense. As a group however... eh. Statistically unlikely. Same for having all orientations represented in a group of 8 people (DA:I). To me it doesn't feel realistic and more like a buffet.
I would love to see a RPG with a female lesbian protagonist, as long as she is a believable character and not just some cliché. Which is sometimes harder than one might think. I tried writing a novel with a lesbian protagonist and struggled a lot to give her a happy end. I think it's already ingrained in our culture that the different ones suffer more or die first and often don't get the happy ending they deserve. It's definitely a trend if you look a movies.
The reason why the main romance is straight is probably because of demographics. Seeing as the majority of their market is in countries who are a kinda irrational about homosexuality, it's no surprise that BioWare is not taking a risk with that.
Also, this is still a fantasy game and not a show piece for tolerance, equality and representation. At some point all this talk about sex distracts from the actual story.