However it’s untrue that in the animal world every species behaves the same in that females are the nurturers and males the aggressive hunters.
I've never argued that (although I think most mammal species do have this kind of division - I'm no biologist though). But I think it's pretty hard to make a case that human males and females are equally aggressive, for example, when near on 100% of violent crimes are committed by males and very few by females. Do you not believe there's any natural/biological/evolutionary component to behaviour then? If not, how would you account for that statistic, which seems pretty stable throughout various societies?
The attempt to equate going to work with ‘hunting’ for instance is ludicrous. Therefore, comparisons to the ‘animal kingdom’ as evidence of why men and women are treated so extremely differently in the world are bound to be weak.
Again, this is not an argument I've made. I also don't argue that it's natural that women are treated differently (badly) - in fact I specifically said I do not believe it is. But I do believe that men and women have naturally different traits, broadly speaking. It isn't and shouldn't be used as a justification for oppression or discrimination or unfair treatment. We can be different without having to be inferior or be treated as such. (Although of course, we are inferior in terms of say, upper body strength, and males are inferior in terms of longevity, for instance - so when we talk of equality, we are talking about political/legal equality, and equality of opportunity. We will never beat men in arm wrestling competitions, no matter how much we are socialised to be tough. But in a modern society, that should not matter, except in sports competitions.)
For me, the sexism comes in with building societies that valorise male behaviours and denigrate female ones. That's entirely avoidable/fixable without having to argue that men and women are naturally interchangeable in our natures. It would be so strange for men and women to be completely alike in nature when one sex is so vital to the process of child birthing and rearing. It seems inevitable to me that this will have an impact on our nature. (Again, on a population level - individuals will align more or less with these traits.)
I think you're making some unjustified leaps in your thinking here, or perhaps I am just not seeing the links. Just because an argument has sometimes been used to support a bad conclusion, doesn't make the argument itself wrong. It's possible to believe that women and men are fundamentally different without also believing that oppression is therefore a fine and natural state for us to live under. I believe we are different, but that it is a gross injustice that we have been oppressed and treated so unfairly. I don't find these to be mutually exclusive beliefs at all.