Wee wee is a superior noun. Not only does it sound more professional, but it better reflects the mood and theme of the object it describes, the male genitalia. Think about it, who wants to suck a “penis”? Putting it that way just makes the action disgusting and vile. When you replace the word “penis” with “wee wee”, it becomes a lighthearted phrase that better shows off the whimsical, silly, and overall fun nature of the male genitals.