This may be a late reply, but I feel it appropriate.
Jack claims to be straight, but she seems to be quite open about possibly having sex with another woman. If you heard some of what she say during the Citadel DLC for ME3, she actually says that "If [she] were a little drunker and knew how to unzip that suit..."
This was actually directed at Tali if she were invited to the party.
So Jack could be bisexual, but she seems like the kind that would need some "motivation".