Complex sentence: The sex/gender distinction is a concept in feminist theory and political feminism which distinguishes the sex of a subject, a natural or biological feature, from his or her gender as the cultural or learned signficance of sex.
Split into simpler sentences: The sex/gender distinction is a concept in feminist theory and politics which distinguishes the sex of a subject from his or her gender. In the distinction, sex is cast as the natural or biological feature of the subject in contrast to gender as the cultural and learned signficance of sex.