Why did humans evolve big penises but small testicles?

We have the penis of a monogamous primate yet our body sizes suggest our ancestors slept around a lot.

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Why DO humans have such large penises?

Nine of the Weirdest Penises in the Animal Kingdom

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Pan (genus) - Wikipedia

Why did humans evolve big penises but small testicles?

Why humans have big penises but small testicles

Human evolution – Berita, Riset, dan Analisis – The Conversation – laman 7

Why is the practice of monogamy widely accepted in our society even if our closest cousins, chimpanzees, and gibbons do not follow this rule? - Quora