What does it mean when a heterosexual is always flirting with a bisexual? And they know you're a bisexual?

How should I respond to a married friend’s flirting? I have an unhappily married heterosexual friend who has flirted with me since she discovered that I was bisexual. I consider her off limits because of her marriage status and her sexual orientation, but she seems to want to be closer to me. Why is she flirting with me and what should I do?