First of all, it is not appropriate for a teenager to have a member of the opposite sex in his/her room with the door closed. However, if your son wants to have sex, he is going to find a way to do it no matter what you do or say. I would not have told him that sex is fun but rather scare him and inform him about all of the sexually transmitted diseases that he can catch. Show him pictures to really add to the shock value. If that does not work, discuss the issues with his pediatrician and then have the pediatrician discuss the matters with him in private. He is too young to be sexually active. Unfortunately, many teenage girls are pressuring innocent teenage boys into activities that are for adults. Please keep a tight hold onto your son. He will thank you in the long run.
If this had helped, please ACCEPT. Have a great day!!!