Probably related to hidden estrus.
https://en.wikipedia.org/wiki/Concealed_ovulation
A likely reason for both being to keep men around so they can protect the woman from bears and other wild animals. Look up the wiki for other hypotheses.
Of course, to get a sure answer we’d have to ask the alien astronauts that Intelligently Designed us (and if nobody did, we’ll just have to keep on guessing).
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