People will defend the most batshit insane things just because they’re used to it.
But I also think there’s an element of (understandable) insecurity to it.
If they concluded that mutilating the penises of babies is wrong, then that makes their penis “wrong”, and society is really weird and judgemental about penises. There’s a huge amount of pressure applied to men about their genitals.
We constantly talk about big dicks and “big dick” energy. Casually saying someone has a small, soft, or ugly dick is seen as a scathing insult, we constantly mock people for it, both in life and in media. Comments about their penises is something used to build up or knock down men. It’s used to make them feel powerful and manly, or weak and emasculated.
It’s no wonder people rally so hard against those who want to see an end to male genital mutilation. The very victims of it typically don’t want to feel like their dick is “wrong”, because society at large has told them that if their dick is bad, they aren’t real men.