Unless you have a vested interest in the person's sexuality (for example, you are interested in dating them, or MAYBE if you're thinking of trying to set them up on a date), then the question's inappropriate. Why does it matter? If you're asking because they exhibit stereotypical traits (such as the liking Glee example from above), shame on you for thinking that there should be such a thing as stereotypical gender traits.
If you're already dating them, you should have your answer already - and being bisexual is a valid response too. Sexuality isn't an on/of switch, and someone can be attracted both to you and to the opposite sex. A better question to ask might be, "Are you attracted to me?" If their sexual preference is an issue, they have an opening to bring it up.