I don't really think he understood that it was rape at the time. He was drinking with a girl. She was so drunk she was almost out of it or probably was out of it. Nate still decided to have sex with her. (He had already had sex with her some times before.) And then he invited his friend to join in.
The girl later said she was unaware of what happened and didn't consent to this. Young men need to know that having sex with someone who is too intoxicated to clearly consent to sex is rape, too.