Wow, I didn't know the gender gap in primary and secondary education had closed so much. That's incredible.
But <10% on the GOP side.
The short answer is: there is still a significant part of the electorate which prefers to have male representatives, and various networks and institutional structures in some parties which give men an advantage.
it is true if Congress would be engaged in sex related questions only
But if it were true it would suggest that there are some important questions going unaddressed the way things are right now.
Assuming you're naive, here's an example from today's paper: https://www.nytimes.com/2018/12/31/opinion/childbirth-injury...
More generally: why have more than one representative at all? Because every person has a slightly different perspective based on where they're from. The wider the variety of opinions (at least to the degree they reflect the diversity of opinions -- urban/agricultural/wilderness; male, female; different levels of education and background; etc) -- the more useful the result.