...because we are supposed to LIKE women who like sex...
Getting into a woman's vagina is a resource. If a woman has sex with a lot of men over a relatively small amount of time, then that lowers the scarcity of getting in her vagina, and this lowers her value in the sexual marketplace. If a man has sex with a lot of women over a relatively small amount of time, then that demonstrates that there is a high demand for his dick in vaginas, and this increases his value in the sexual marketplace.