As far as I know there is nothing in the game which indicates this one way or the other, but I don't doubt that people have strong views on the matter, so express them here. Some sort of explanation for your view is, as ever, superior to the choice alone.
She has a talk with Haer'Dalis in Throne of Bhaal that indicates heterosexuality. Something about Haer'Dalis telling her he wished he could comfort her etc etc, sweet talking her in his charming way, and she seems quite touched at a point, due to the classic use of "I..."
Ofcourse, if anyone wants to over-analyse anything, there's that bit where she says she dreams about dryads - meaning there could be a bit of lesbian in her if she's dreaming of women.
Something about that dialogue doesn't convince me though, I can't place it, but I say she's heterosexual.
But if you wanna make her bisexual, do so. I'm kinda nummed to arguing over peoples' sexuality nowadays, and there's so many fanfics out there that make her involved in lesbian stuff. Most are done badly though, but I'll keep my opinions on fanfics outta this