That's part of the humor. The exaggerated stereotype.
People are different. To me, very few things sound more repulsive than French and it doesn't matter how sweet the person is.
their guttoral rhotic consonant.
They are very similar but it doesn't sound foul to me in German, it only does in French. I guess that's because of the other factors that create its whole environment.
You're wrong. It's because of their phonetic system - their diphthongs, their method of vowel articulation and their guttoral rhotic consonant. I wouldn't be as arrogant as to project my aural perceptions onto individuals, let alone whole nations.