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Younger Male, Older Female... maybe it's the allure of experience, refinement and beauty toward the zest of youth, curiosity of inexperience and in some case raw physical attraction.... or just a simple Oedipal complex
I don't know, but you are right, in many (if not all) of those couplings previously mentioned both partners seeking something that only the other one seem to fulfill. Wether it be a sempai, a sensei, a maternal figure... theyre all human, only the perception of 'who' they are is what makes them even so more atractive compared to other prospective interests.
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