A female is in demand while a male is not because her egg provides nutrition for the offspring while the sperm doesn't
From The Selfish Gene
Is there anything a female can do to reduce the extent to which her mate exploits her in the first place? She has a strong card in her hand. She can refuse to copulate.
She is in demand, in a seller’s market.
This is because she brings the dowry of a large, nutritious egg. A male who successfully copulates gains a valuable food reserve for his offspring.