A man shows up...
to a Halloween party and knocks on the door.
The hostess answers and tells him that he must be in costume to get in!
Dismayed the man starts walking home, stops, strips down to his undies and goes back.
He again knocks on the door and the hostess answers, looks at him and says "I told you, you have to be in costume to get in!"
He says "I am in costume".
"What are you then?" she asks
He says "I am a premature ejaculation, I just came in my underwear!"