I guess relative is relative, but gender occasionally being used as a synonym for biological sex in humans was still a pre-modern thing. When you name your grammar classes masculine and feminine it's pretty natural for someone to reverse engineer that construction in their heads and start applying it back to people when they're trying to sound fancy or are squeamish about saying sex.

It's the split from biological sex into its own category of bullshit that was partly Money's depraved contribution. Which in turn lead to a vastly increased usage.