I wouldn't pursue a long term relationship with someone that goes to bed with the first guy available, especially if she tells you that she doesn't know why she is attracted to you. Her libido is fine, even desirable; her brain, or rather her emotional stability, is not.
If this story did happen, just move on. There's better avenues to meet the girl of your dreams. Pursue your hobbies and go to places where people with similar interest go. That's where you are more likely to find a reliable match for you.
By the way, get rid of that stupid idea " girls use us". They don't use us more than we use them. Or males use other males, or females use other females, etc, etc. If you feel used in general, then there's a problem with your self esteem. In a way, you and the girl of the story, in a very ironic way, are a match. You both have some problems.