"...current social scientific and feminist thinking, which regards gender as social construct and sex as biological..."
This is yet another example of a specific discipline, who defines its terms in a very specific way, being surprised when the general public don't consider terms to be as specific as they do. In general usage, "gender" and "sex" are much more interchangeable in terms of their meaning than they are within social science and feminist disciplines, in my experience. The tendency in general language is always towards the non-specific, just as it is always towards the specific within specific disciplines.