Well obviously "to bang" means to have sex with. "banging" is a term where someone is in the process of having sex. Justin is a common male name. Can also be for a female if your parents were on crack when you were born. So in the term "banging justin" you are in the process of having sex with justin. One situation where it would be difficult to bang justin would be when you are at your friends house with justin, your friend is home with one of there friends (4 people, stay with me), you and justin are on a couch, your other 2 friends are on the ground in front of the couch, and you are wearing a unitard.
Attention: This is a hypothetical situation. This event, even though thoughroughly described in detail, did not actually happen.
Save a drum, bang a justin.