Re: Is this just an English thing ?
In my experience, "gender" was, until relatively recently, commonly considered merely a less titter-worthy synonym for "sex".
I'm guessing you're young-ish?
Until recently differences in sex (male/female/etc.) were referred to as "sex", and "gender" was reserved for the grammatical difference between masculine and feminine. Then Americans got too prudish to say "sex" and had to find an alternative. I'd date it from around the 1990's.