Competition strategy. The more people you can convince that they shouldn't hit on women means fewer people hitting on women, which improves the chances for those who hit on women.
Same reason 'slut shaming' is a thing, which is the same thing, just from the other point of view. The more people you can convince to not accept the advances of others means more opportunity for those who do.
Same reason 'slut shaming' is a thing, which is the same thing, just from the other point of view. The more people you can convince to not accept the advances of others means more opportunity for those who do.