"A hundred men and a hundred women."
Translation:Stu mężczyzn i sto kobiet.
Well, 'what is going on' is a bit too general of a question...
With this sentence, which generally as a whole uses Nominative, it's as follows: men take the special form "stu" as it's a 'masculine personal plural' noun, women, as they are in the 'other' (not masculine-personal plural) group take the basic form "sto". The special Nominative/Accusative form is, I believe, always identical to the Genitive one.
And like majority of numerals, "100" takes a noun in Genitive. It's like "A hundred of men and a hundred of woman".