You probably made her day when you asked her out. You did the right thing by asking her out because you know what's up. She's interested; however, she is involved. Let's examine your choices...
1. You can put your life on hold for this woman and ask her out again once she breaks up with the guy. She may or may not be as receptive as she once was.
2. You can try to steal her from her BF. But think about karma with this one. You wouldn't want somebody to steal your GF from you now, would you? A cock block is a cock block is a pussy block.
3. You can look at the positive from this experience. If it wasn't for her BF, she'd take you up on your offer, stud. This means women find you interesting! There are numerous women walking this earth and if you can attract one, you can attract more.
Why place your life on hold for some woman you don't know very well with this new found confidence you have? There is a chance you and her may not gel together in the longterm.
I'm not going to tell you what to do, but I will share with you this bit of information that has saved me stress with women and dating. For every 1 attractive woman you meet, meet 10 more women and then see just how attractive that 1 attractive woman you met you think is.
