Because females are the narrow part of the flow chart. You know the old joke: You can't get a baby in 9 months by putting 9 men on the job?

Sci-fi authors for many years have based 'moving humans to a new star' by assuming that a colony ship would need X times more women than men.

If anyone ever took the idea seriously... sure.. do something with men also, but hormonal contraceptives are already worked out for women.

... and I don't pretend that *I* have the only possible solution... just that all the others I can think of are pretty extreme.. Soylent Green, anyone?