A guy walks into the bathroom and sees a very small man taking a leak. The little man looks at him and says "Hi! I'm a leprechaun! And because I like you, I'm going to grant you three wishes."
The man thinks for a moment and then says "I'd like a beautiful house."
"Granted. When you return home, you will have a gorgeous mansion."
"Great! Now I'd like a beautiful woman."
"Granted. When you return home, you will find a woman so amazing you will never look at another woman again."
"And I would love to have a huge penis."
"For that, you'll have to let me screw you in the ass."
The man hesitates, but since he wants a huge penis, he consents. As they're going at it, the man says "I can't believe I'm letting a leprechaun screw me in the ass!"
The little man says "I can't believe you thought I was a leprechaun."