I could see introducing fixed orientations for the characters later on. But with five NPCs, I can see why they didn't want to do it now. I mean, I recall people were really upset when Dragon Age: Origins came out and there was only a SSM male romance option...and it was Zevran.
Since we only have five companions right now - and only two women - eliminating playersexuality would cut down on romance options considerably. But if we go up to 8-12 companions eventually, having a mixture of gay/bi/straight companion romance options makes sense.