Given that sexes are, nowadays, considered to be equal… the advent of gender seems to be somewhat unwarranted.

I’m afraid things are not so simple.

First of all, sexes are only declared to be equal. Nobody – or, more exactly, very few, actually consider them to be absolutely equal.

Secondly, they are only of equal importance. Not at all ‘equal’.

First things first.
We, all, have been raised seeped in ‘culture’.
And almost all contemporary cultures consider women to be ‘second’ to men.
Hence both men and women – both raised by the same ‘mother’, have an ingrained bias towards men being ‘somewhat’ more important than women.

Last, but not at all least, sexes play different roles. Biologically, socially… any way you look at it, men and women do different things in order to fulfill their jobs. OK, those roles have been ‘blurred’ during the last decades but their are still discernibly different.
But of equal importance, mind you!

Just think of yin and yang. Would you say they are equal? Could you say which is more important?

Hence gender!
‘Genders’ do exactly that. They underline the functional differences between sexes while demonstrating the equal importance shared by both sexes.

And, last but, again, not least, genders teach us that only the biological roles are different AND fixed.
Social roles are also different but they can be fulfilled by either sexes. Men acting as secretaries for women bosses, women leading households – both income-wise and/or as ‘the pillar’, fathers using their maternity leaves, women flying fighting jets, men happily working as nurses. You name it!

You get accustomed to it.
Study genders. It will come easier.