When discussing difference, it is important to have an idea about the different kinds of difference.
Let me distinguish several types. This is not an exhaustive discussion
a) Categorical difference.
You can insert every relevant item into one or other category. The things inserted in each separate category have little in common.
However, there is usually always a level at which everything has something in common, and something that differs… so this is harder than it looks. For example, horses and rats both have four legs, teeth, two eyes, two ears, tails, give birth to live young, and so on, and yet we all usually agree about which category any particular specimen will go into. Horses and sharks may also have things in common… although the similarities may well be less striking, but you can probably think of similarities if you try hard enough.
b) Binary categorical difference
Not only are the things supposedly easy to insert into categories but they are also classed as ‘opposites’. Usually, but not always, men and women are inserted into such categories. In our culture men are supposed to be aggressive as opposed to passive, not be obsessed with appearance, be non-maternal and so on. Some forms of feminism accept categorical difference, or even binary difference, and argue that women have particular abilities and understandings which have been ignored or condemned by the patriarchy. Given the enforcement of categories this is likely to be correct at the moment, even if not ultimately true.
c) Statistical difference
This is were it is recognised that the categories are actually a bit fuzzy. For example, men are generally taller than women. However, very few people would claim that all men are taller than all women. I suspect that most, but not all, gender characteristics are like this. Consequently, the more rigidly the categories are enforced the more people may feel they do not fit in. Are tall women actually men? Are men who like children, really women? Silly yes?
d) Culturally enforced difference
In this case, what are declared to be categorical differences are made absolute and enforced, and such categories occupy places in a power system. The rich are better, hence they get education and this proves the poor are ignorant and stupid and so on. Men get encouraged in violence, women discouraged.
e) Taboo categories
These are reserved for those things/processes which do not fit socially important categories. Cloven hoof animals, transgender people and such. These things are often defined as unclean.