Fair point. If your argument is that physiological factors matter, but gender is an insufficient variable to segregate on then I don't disagree.

I think it comes down to a matter of practicality. In most cases is gender a good enough heuristic? Maybe, or maybe not. I don't really know, but it's probably one of the simplest variables to consider. Perhaps it would be better if a more complete (but complex and harder to measure) set of factors were considered.