A douchebanger is a
girl who repeatedly has sex or hooks up with complete douchebags. She may be slightly aware or completely in denial. She does it on a regular basis, and has not been well exposed to other types of men...
normal guys who have manners,
normal taste in clothing, and know how to treat a lady
"
Wow she's such a douchebanger, I can't believe she's going home with that douche-sack."
Girls who usually have
low sel-esteem or think they know what is "cool" sleep with douchebags. Therefore, she is a douchebanger.