Wow! I just can't unsee that pelvic thrust Gray gives as she sells the song. Jeannette McDonald wouldn't be caught dead acting like that. She's way too classy, from her clothes to the style of her voice and musical genre. Eddy and McDonald are the types that the Dead End Kids would beat up in an alley.
But the purpose of the code was to reinforce proper, moral behavior and make it look desirable, and who better than Eddy and McDonald? Even their chemistry is whitewashed to mildness, not the burning passionate can't-keep-my-hands-off-you stuff we see in some 30's films. Hollywood wanted us to grow up to be them: good, decent, empathetic, reliable, and hard-working "salt of the earth" types.
Thank goodness for their senses of humor, which made them human. Otherwise McDonald would be too unapproachable and people would hate her, and Eddy would be too much a goody-goody, and women would pass him by for a bad boy.