To think that 'it' is a perfectly good pronoun for an animal

[RoseDeGambrinus]

...and want to explain some basic biology to people who talk to DC as if every single cat, dog, squirrel, insect, even chicken or cow in the world is a 'he'.

At a basic level, yes.

However, all cats are "he" to my toddler because our cats at home are boys. And obviously cows/sheep/chickens are called "she".

YANBU. I met with a friend for a dog walk, and her friend came along too with her dog. My dog was pestering hers so I told him to "Leave it" - his general command to get him to stop sniffing / licking / eating / annoying. She got very uppity - "She's a she, not an it!"

She may well be a she, but the command my dog knows is "leave it", not "leave her"

My poor fluffy chihuahua being an "it" would be a step up, he's usually referred to as "she" despite still having his balls! My staffie is referred to as "he" because she looks more butch I guess.

I politely correct people if I'm talking to them (they say "oh isn't she gorgeous" I say "yes he is", without the passive aggressive emphasis on HE btw!)

I wouldn't call them "it" myself but no problem if someone else dosz

I suppose I wasn't thinking so much of the etiquette of pet animals with their owners, more the labelling of completely random animals as automatically 'he', never 'she'. Trivial, I know...

No, I get irrationally bothered by the automatic assumption that all creatures are male, although only since I've been talking about them with my son that I have noticed how frequently it happens. I once spent a day saying 'or she' every time my mum referred to an animal using male pronouns (in a mostly lighthearted manner) to demonstrate how often it actually happens and how instinctive it seems when you do it all the time and we were both surprised (and infuriated!) at how many times it came up.

I get mad at anyone calling a pet It. Pets are members of the family, not objects. Maybe ok for worms or flies, never never for cars or dogs.

This reminds me of my elderly god mother who insisted all dogs were "he" and all cats were "she". She never had children herself. Maybe she never discovered the relevance of a species needing both male and female equipment.