Maybe it’s only advantageous if u spend most of your time hanging from a ceiling by your toes

So why might the serotine bat have evolved such an enormous sex organ? The authors suggest that females might use their tail membranes to avoid mating, and the males' long, thick penises could be used to push that membrane aside. "The hollow structure observed in the dorsal side of the erect penis might serve as a suction cup and support the maintenance of the copulatory contact," the authors wrote. "Therefore, the penis's size, shape, and prolonged embrace might further support the transport of sperm cells into the vagina and through the long cervix."

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This only partially answers the question of why, though.