Added to this biological differences can lead to differences in behaviour, the link between different levels of testosterone and aggression for example.
That is sex, not gender. Which is why I don't want males in women's spaces regardless of the "gender" as which those males identify.
Gender, on the other hand, is telling people what they ought to do, either because of their actual sex, or, nowadays, because of their actual sex AND because they have failed to identify as the other sex. We do not need that. We do not need to make it more inclusive, we can just do away with it.
No decisions are made taking into consideration male aggression, and never have been, so while that might be beneficial, it is not what is done today. (Remember that thread about the article where a woman said she wouldn't let a male babysit her children, based on statistics about males committing sexual assault much more often than women? Remember the outrage of mumsnet posters about people basing decisions on observable differences between the sexes?)
Sex segregation in bathrooms and prisons is, officially, not because of male aggression but because people don't want to be naked in the presence of the other sex.
Again, nothing to do with gender.
Behavioural differences between the sexes (not genders) may or may not be inherent, but while there is a need to acknowledge them, there is no need to enforce them.
In short, women need safe spaces where men don't have access, because male aggression exists (whether it is nature or nurture doesn't matter, the fact is it exists) but there is no need to socialize boys into being aggressive. It is utterly bonkers that society is currently doing exactly that.
That - socializing boys to be aggressive and entitled - is gender. Why would anyone want to keep that?
(The only other meaning of gender there is is grammatical gender. I think we could do without it, many languages do, but it would be a pain in the ass to change, so not high on my agenda to abolish that)