Why, in theory, isn't Congress made up of 50% women + 50% men?

Among Democrats, women are now up to about 40% in Congress. I could easily imagine that passing 50% in another decade or two as older members retire.

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.

Part of the answer might lie in seeing statistics for what percentage of congressional candidates have been women, over time.

historical inertia?

> isn't Congress made up of 50% women + 50% men?

it is true if Congress would be engaged in sex related questions only

I don't see why that follows.

But if it were true it would suggest that there are some important questions going unaddressed the way things are right now.

It was kind of joke, but why would congressman’s sex affect his decisions if he is professional? What are these important questions which are so affected by Y-chromosome?

I don't know why you're being voted down -- perhaps people think you're trolling.

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.