Is it illegal for someone to post a sex video on the internet without consent of the other person and if so what is the plenty?
A old friend has left horrible voice messages threatening me to put my sex video on the internet if I do not continue to see him what can I do to prevent this and if its illegal?
In general, it is illegal to publish or distribute material containing the image and likeness of a person, without that person's consent. The actual laws relating to this "right of publicity" vary from state to state. If you were the person who filmed the event, you would be considered a copyright holder as well, and may have rights to pursue a federal copyright infringement claim. Any media outlet that publishes the material can be responsible for publicity right and copyright violations as well. There are limited exemptions to these rights such as "fair use" and "newsworthiness" under the First Amendment. However, publication of a private sex tape, as described above, would not likely be protected under these legal defenses. A court can award substantial damages and possibly attorneys fees against any person responsible for publishing or republishing the material. It can also enjoin (i.e. 'stop') the continued publication of the infringing content.
In my opinion this person has committed a crime if they are threatening you with publication and will refrain only of you have sex with them. There are privacy laws, but many of them apply only in the ircumstance of commercial behavior. You should save any recordings where there are threats and tell you friend to relinquish the videotape or you will have him criminally charged.