Is it sexist that mutilating genitals of baby girls for theistic reasons is illegal, but with boys its legal?

Yes, it is but mostly its because its religious that its allowed. If social services learnt that a boy was being told he was not as good as his sister and that men were worthless and had no rights and made to serve his sister and not allowed out of the house but made to stay in and clean etc they would cry emotional abuse. However if it is a girl who is treated like that and the family explained that this is part of their Muslim values, this would be allowed to continue. Laws exist to protect food animals from unecessary suffering but if Halal and Kosha butchers are allowed to be excepted from it because according to Islam and Judaism animals must slowly bleed to death.