It has to do with reproduction. The primary purpose of life is to reproduce, this is why we have urges to have sex. There is something called the "Fundamental Asymmetry of Sex" this is the asymmetry between possible reproduction "rates" between men and women. This is because men can have multiple children with different women. While a woman can only have "one" child at a time. For a woman to make a baby, it takes around 9 months. For a man to make a baby, he can "make" one every few hours. For this reason, women have to be picky on who they choose to sleep with (genetic quality). While men cannot be picky as to who they sleep with.
TLDR: There will always be men (multiple) willing to fuck a woman (singular), but there will not always be women (multiple) willing to fuck a man (singular).