The Semen Demon will randomly appear while you masturbate. He will collect semen and store it in a jar. Once this jar is full, he starts shoving semen down people's throats in an attempt to kill them. If he fails, he will force two people to have sex with their bedroom door open. Here are some tips to keep him away: 1. DON'T!! He has a 97% chance to appear if you try to keep him away!
Also note that the Semen Demon is covered in semen, which means that he could just be a serial killer wearing a semen covered costume. There have been several reported sightings of several Semen Demons walking around. This can be chalked up to just some sort of illusion. The Semen Demon's reasons for doing all of this is unknown as of now.