Because anything that lowers a womans reproductive chances / sexual market value is a threat, and all women are theatened by something like this which is why they all rally round against it.
Slut shaming is a similar one, but more complex - women encourage other women to be sluts and that its fine, but are cautious because they know going too far will cause themselves to be used goods to other men. So they take great pains to hide/lie about their own "count", whilst encouraging other women to be open about theirs. They are also the first ones to slut shame another woman if there is an immediate gain from it (making themselves more attractive to local chads at a party for example).
