here is no hint showing she is bisexual or lesbian.
There is, however, no hint showing she is NOT, either.
Let's put it this way :: THERE IS NO PROOF IN THE GAME EITHER WAY. If someone can prove me wrong, I'll be gladly indebted to them. But until someone finds somewhere in the game where Imoen states "I do not like girls in any romantic way", it's not correct to stay she is totally hetrosexual. Bioware left that up to fan interpitation. There's no proof.
It's not fair to decide that just because someone doesn't say outright that they are bisexual or gay, it doesn't mean they aren't. There are very few people that run around the streets screaming their sexuality.
Personally, I think it depends on what sort of experiments were preformed on Imoen. From the way it was talked about, I got the impression that anything would have been done to try and get her Bhaal abilities to react. If this is true, then it might be very likely for Imoen to rather NOT have any sort of romantic relationship with men after what she had suffered. That's just a disturbing issue to get in to, and one I would rather avoid, but it's possible.
I don't think Imeon would go out of her way to date boys, or girls. I think for Imoen, she would be looking for that 'special' person, no matter what sex. I would find it out of character if she refused a 'soulmate' because they were of the same gender as her. She seems like the type to throw herself in to a relationship, and I also think the aspect of a homosexual- 'forbidden' relationship- might appeal to her. I got the impression that the 'love defeats all' motto would be very important to her.
I think the bigger issue isn't gender or race, but the plain 'would she' issue. I personally don't see Bhaalspawn relationships as incest, but Imoen clearly sees you as her brother/sister. Isn't that more of an issue rather then gender? Would she be able to get over that? Or would she even want to get over that? Maybe she's perfectly fine being your sister/brother and best friend.