It’s not as simple as that, at least for humans.

Firstly, you’re not generally going around looking at people’s genitals, and even if you were you wouldn’t be able to tell if someone’s had surgery.

Secondly, while sex and gender are separate, the distinction is often brought up without nuance. Sex is not a simple concept. A trans woman has neither the hormonal levels, body fat distribution, strength, brain mapping, nor (necessarily) sexual function of a cis man. It also erases all sorts of intersex conditions. It’s impossible to define sex in a way that doesn’t misrepresent some class of cis folk.

We assign pets genders because of our human experience, but they almost certainly lack the concept of gender. A dog doesn’t care what you call it. Hell, a dog rarely cares what it’s fucking.