sex with a ghost

A traveling paranormal expert is giving a lecture at a local university.
The talk turns to contact with ghosts and he asks:
"has anyone here ever seen a ghost?" About 50 people raise their hands
Then he asks: "Has anyone here ever talked to a ghost?" Around 20 people raise their hands.
The expert then asks if anyone has ever touched a ghost, ten or so people raise their hands.
Next, having his interests peaked, he asks the class if anyone has ever made love to a ghost. One man in the back raises his hand.
"Really sir", says the expert, "Would you mind coming to the front of the class and talking about your experience?"
The man says sure and walks to the front of the class.
"So, tell us about when you had sex with a ghost."
The man looks confused, "A ghost? I thought you said 'goat'"