I find it pretty amusing that the "realistic" excuse is only ever brought up when it's about sexuality.
Apparently people have no problem with a mine full of dragons, but four bisexual people in one city is too much!
All fantasy universes are very closely based on our own world and our own human experiences. Even the dragons for which the series is named, are based on something from Earth. Dinosaur bones were the origin of dragon myths. Because of the fact that all fantasy universes closely mirror or own, its not unreasonable to also assume that heterosexual and homosexual people would exist in them as well, just as they do in the real world.
Now of course Bioware could have decided at one point that everyone in Thedas is bisexual and that would then be canon for their world, and no one could argue with all of the companion characters being bisexual or playersexual. But just as bisexual people wouldn't like hearing that 'bisexual people don't exisit in Thedas,' you shouldn't expect heterosexuals (or homosexuals) to be okay with people resembling themselves being written out of the universe either.
Also it is worth noting that we all want various realistic elements in the series, we just vary on what is necessary and what isn't. I'm sure no one would be happy for example if gravity didn't exist.