If someone is threatening to divulge personal information of a damaging nature unless you 'go out' with him would that be considered a 'criminal act'?

Most people would call that blackmail, and could also fit under defamation - either way it is certainly illegal. Tell him that you will not change your mind and inform him that you intend to file a police report if he doesn't leave you alone; you should probably file one anyway, just in case he takes it the wrong way and gets too serious.