I know exactly where you're coming from! PinkPenguin06 is right on the dot, but I'll add some of my personal experience.
It's hard: You generally find yourself more attracted to guys, and a great girl comes along who you find yourself enamoured by... except she's taken, not that it doesn't keep you from hoping. Which gets painful and slightly embarassing.
She is in a relationship; it may be/become an open relationship, but would you really be comfortable with that? They certainly can work, but it seems in your position that it might be ok at first but then you'd get upset, finding yourself wanting more.
Thing with same-sex crushes that I've realized over time is that, wow, if I thought finding guys who I was truly into who had mutual feelings was difficult, it's even harder finding women that way! Plus, the attraction being a new or rarer thing, it seems like a much bigger deal to have not work out than a heterosexual crush.
But this scenario clearly seems that it would just lead to disappointment on your part. Enjoy the crush while it's fun, but try to realize that something actually happening probably would be painful for you. Not to mention that, as appealing as she may be, it seems that she sort of enjoys exploiting your crush as you say she's fully aware of it. So, when it starts hurting, try to maybe distance yourself a bit for now.
Thank you for your post: I'm actually experiencing a version of this myself right now (I'm done with college and all that, you'd think that I would learn to avoid my mistakes. ) and your story helped me put my own situation in perspective.
And just to add, thing is with attraction... you or I can talk our ears off about what our "type" is, etc. etc. But then we meet someone, male or female, who is that type and we don't find ourselves clicking. Whereas we might meet someone we'd never think we'd be into only to find ourselves really attracted and interested in the person. (As you described here. Attraction is such a crazy, random thing!)
Over time, you will meet more women (and men) who are really "wow" to you, and fun dates or even a good relationship will result. And it's certainly worth waiting for someone good who's reciprocially available then to see yourself get hurt.