I answered Isabelle's ad for a guy to help her with her sexual studies theory. She wanted to prove that men like to get fucked, and she wanted me to prove it by fucking her step-brother in the ass! And damn was Ben surprised when she told him what I was there for. But he was a good step-brother and hopped on my dick in the name of SCIENCE! Before he knew it he was bouncing on my dick happy as hell to prove his sister right. But he couldn't resist getting turned on by her watching, and begged his step-sister to let him eat her pussy out. Learning is fun.