I'm trying to imagine how one could accidentally get fucked in the ass. Like, he was trying to move his friend to a better position and accidentally unbuckled his boys belt in the process while reaching around his waist to move him. Then his pants fell down because of the position. The guy helping him was already in his boxers because he was ready for bed and his dick slipped through the pee hole when he was trying to lift his friend. The friction from the back of his friends shirt causes him to get an erection. Then while standing up after all this he slips on a bar of soap that fell out of this guys bag, his erect penis hits the band of the guys underwear pulling them down, and his penis manages to slide right into the guys ass which is naturally lubed with sweat because he had so much to drink and his body is trying to detox. Seems like a realistic scenario to me so I guess it could happen to anyone.