By age 10, it is important to have begun discussions about his sexuality. Curiosity is a normal part of growing up, and children tend to "share" a lot with their friends at this point. There are some excellent books available to help your son understand the differences between girls and boys and to help explore his feelings about himself and others.
I think the bigger question about the movies is where he got them. . . if someone from school or on the playground shared them, do his or her parents know? Or worse, did they come from an adult? If so, that needs to be handled immediately. The other question: did he actually even see them? You may have found them before he had the opportunity to view them.
I don't think it is appropriate to punish your son for this "first" offense. Sexuality is a normal part of life. That said, he needs to understand what is and is not appropriate behavior, that it is essential to respect both himself and members of the opposite sex, and that like the cartoons and fictious movies he watches on TV, these movies and materials are fantasy stories, not really the way people behave.