Why don't you go do it? You want me to do it! Hurry they are waiting If you knew why I do it . . . I don't care why you do it! To think I admired you For now I despise you You liar - you Judas You wanted me to do it!
Julius Caesar: Act IV, Scene 3, in Brutus's tent. Brutus asks his page boy, Lucius, to play him some music on his...er...instrument.
There's one scene early in The Light in the Piazza where Fabrizio looks down aghast at himself, motioning towards his legs. He's actually referring to his clothes, but the innuendo becomes even greater when one factors in the relevant line in Italian, where he says "She can't love a little boy!"
In Me and My Dick Weenie gives Joey some homemade soup, causing the audience to gasp. They then clarify it's just chicken noodle.
"Now a pussy's good for maybe six or seven at the most... and I'm sure they can't compare as far as taste!"
Made even more hilarious by the fact that the entire song is just a Hurricane of Puns, and yet this one was completely unintentional (it was referring to actual cats, and how much meat you can get out of them).
There's another one in the second act, in Mrs. Lovett's piece By the Sea. She's talking about how great life will be once she and Sweeney Todd are living by the sea, and saying that it'll just be the two of them and the occasional paying guest, and if he feels like it "now and then you could do the guest in". She talks about getting married and tells him to "bring along your chopper", meaning his razor. However, Sondheim notes in his book Finishing the Hat that when the show premiered in England, he discovered that "chopper" is also British slang for penis.
One of the major settings of Tannhäuser is the "Venusberg," the hill of Venus. Richard Wagner was aware of the potential for gynecological jokes and refrained from making it the title of the opera.