For a good work of art, a good artist and a worthy focus are both absolutely prerequisites. Thus, if a poet says "your bonnet and the person below it inspire me for a good sonnet", it can be seen as a charming compliment. But in a situation of immediate erotic interaction (or in a flirt, if you want it for $5 less), the idea of sitting down to count syllables is quite inappropriate, unintentionally funny. Moreover, both Fred Astaire and the character he plays have their poetic power in their legs. A high price to pay for a rhyme, but Hollywood and Broadway could afford it.

By the way, like many other art forms once held sacred, the technique of writing sonnets (by Italian or English rules) descended to a pure sport during the 19th century, organized as contests by newspapers, similarly to crossword puzzles. Poets who valued their originality could no longer join that crowd. Without those strict rules, the effort of being a truly outstanding poet increased.