A:They both have a one-in-a-million chance of becoming a human being.
Because dogs can't whistle. (X-post from r/dadjokes)
Because they'll bite us!
Ello, 'ello, 'ello!
A minister! Courtesy of a patient.
A: So he can tell if he is coming or going.
A: So that sperms can at least enjoy the scene even if their entry is Restricted!
A sperm as a 1 in a million chance of actually becoming a human being.