What Chrys said. As far as I've heard, Solas makes sense, Cullen doesn't. I'm pretty disappointed for my female dwarves; I'd rather them have put the effort into making Cullen accessible for them than IB, but oh well. They seem to have their own ideas about what people want, and I'm completely stumped on where they're getting those ideas for female dwarves

Well they got m/m fans right b/c I'm like the only person in the world who's disappointed 
With Cullen I'm sure their "reason" is "he liked elves and humans in DAO." And they could even use the excuse that he didn't respond to males in DAI for why he's straight, too.
EDIT: I'm not saying either of these are good enough resaons, and in fact I don't think they are. But I'm just saying that's what they'll say.
For Solas, well, I'll believe that elf-only reason when I see it (like maybe copulating with a dwarf will make him explode... I don't know xD) but do they give reasons why he's only into girls? Like, I don't know... he once dated a guy who tried to cut his balls off so he's like, never again? 