I give no reasons because the burden of proof is on the one who claims one culture's judgment is more valid than the other's.
You aptly demonstrate that all cultural judgments are not equal. Cutting off a little girl's clitoris is wrong even if Egyptians think it's a great idea. It is by no means a "given" that two different cultural judgments are equally valid.
I claimed not that all cultural judgments are equally valid, only that the following two cultural judgments are equally valid:
They're equally valid because they're mirror images of each other, and neither has superior internal consistency to the other. They're similar enough that the burden of proof is on the one who claims that one of these two judgments is more valid than the other.
Here's another example:
One year, as a free lancer, I wrote an RPG adventure. For the nonplayer-characters, I determined the gender of each one randomly, whether they were allies, villains, or victims. I didn't want to assign the genders intentionally because I didn't want my subconscious biases to show through.
The designer the RPG for which I was writing the adventure reviewed the manuscript. His comment was, "Why are the player-characters always beating up chicks?" In other words, he was astounded that I had so many female nonplayer-characters as villains. He thought it was misogynistic to have the characters in combat against women so often.
I intended the presence of female villains to demonstrate gender equality. Instead, it was (in the eyes of at least some readers) misogynistic.
In the same way, if a misandrist RPG designer from an alternate dimension were to design a man-devouring she-demon, she'd intend it as a misandrist fantasy. If the description of the she-demon were to appear here in this dimension, we'd label it misogynist.