She is human, just like the rest of us, and people are naturally not monogamous. What keeps us monogamous is our inhibitions. If she was so consciously willing to do it then its very likely she would have done it to you again (If this was the first time) and will probably do it to her next partner.

"Friends" is just another way for her to get what she wants.

EDIT: Or....it could be one way of her saying she wants a three-way....