"Mom, I'm going out."
"You're not leaving this house until you change out of that miniskirt."
"Why Mom?"
"Because I can see your balls Richard."

hhmmm .....

There's been a similar one down here for years, the set-up is

"Did you hear they ban miniskirts from South Beach?" (Miami Beach)

"No, why?"

Same punch-line as above. Eww.