I can best explain why the Harlem Renaissance was considered so "sexy" by starting with an anecdotal story about Ezra Pound.
When Allen Ginsberg was a very young man, he went to visit Pound in the insane asylum. For the most part Pound enjoyed the meeting, and the fact that Allen brought him thick slab bread and mayonaise. (Pound had a thing for mayonaise on bread.) But suddenly during the meeting, Pound's mood took a sudden downward shift. He buried his head in his hands and murmered over and over, "I am sick. I am sick." Allen asked him if he meant his body or his mind. Pound answered, "My mind. All I do is create great balls of crystal that no one can lift."
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The Harlem Renaissance came on the heals of the Imagist era of poetry. There was some spirit and passion in the Imagist school, but for the most part they became ever more infatuated with their own cleverness, their hidden meanings and literary allusions. Erudite symbolism replaced the beating heart.
The Harlem Renaissance was in a large part made up of less "schooled" writers. They discovered literature, writing, rythm, and passion outside of the Ivy League schools. They were raw. They were gritty. They were colorful, passionate, melodious, and alive... at a time when poetry was becoming a dead voice.
How can anyone not find that sexy?