I heard the most obvious double entendre song of my life on CBC radio about a week ago. The chorus is:

Churn and churn 'til the butter come
Churn and churn 'til the butter come
Keep on pumpin' 'til the butter flow
Wipe off the paddle and churn some more.

Does anyone have the rest of the words? I think the last line in the last verse was "You're my brown cow and I'm your bull".