What do I tell this girl...?

[inside and out]

I met a girl on a dating site (she only wanted to meet new friends), we talked and became facebook friends. We are both in our early 20's. I first talked to her online before Christmas, & since the holidays we havent really talked. A few days ago I wanted to talk to her on facebook, but noticed she had a boyfriend. I sent her a message (that only she could see) "hey how's it going?" blah blah blah
She said she had a bf and had to get off the dating site because he didnt like it, which I understand.

Now I wanted to ask her out, and would like to be with her... but she has a bf. What should I say in response to that?? Should I say something like: Oh ok I understand, can we still be friends?? OR should I admit to her I liked her, and my intention was to ask her out, but if she has a bf I understand?