Some related thoughts have been floating around for a while: I would actually love to remove the “Castrated” field entirely, and instead make it a log-driven state somehow. In other words: rather than setting “Is castrated” on the animal level directly, you would record a log that sets the state to “Is castrated” much like “Location” and “Group membership” is set via log.
The “Castrated” field was added very early in farmOS, and it feels a bit out of place in my mind, to be honest.
Your suggestion to toggle it based on the selected “Sex” does make sense, given it’s current placement, though I agree. It probably wouldn’t be hard to make that hidden/visible via JavaScript if you’d like to give that a try.