A "Semen Demon" is a female who seeks for mens semen in order to have kids or just for the fun of it.
M y ex-girlfriend broke into my house, drugged me and when I was passed out she jerked me off and when I nutted she kept my semen in a jar I guess you can say she's a real "semen demon".
A individual, human or otherwise that is capable of excreting immense quantities of semen. Commonly displayed in various acts of a sexual nature in pornography, specifically but not limited to the Japanese Hentai
John: Hey Matt, you see that dude in A Slave's Revenge?
Matt: Oh yea mate, he's a real Semen Demon.