Why does it matter so much to atheists that God doesn't exist?

IT does not matter, mainly because it does not exist.

These are man made rules - made thousands of years ago for the use of the humans who lived thousands of years ago.

A belief in a god is not the problem.

If English makes 3 additional gender terms to accommodate for XXX, XXY, and XYY people, what would be the most realistic terms for those genders?

THE BIBLE IS THE PROBLEM!

My problem is that people who try to shove their rules down everyone's throats - including by physical force - DO EXIST .

And then use and ENJOY these fantastic scientific advancements every single day of your life?

If a non-LGBT man (of any age) from a Western country attracts far more mosquitoes than potential dates, what does that say about him?

Because these are not any gods rules we are talking about.

I do not personally care if you believe in a god or not.

I am speaking to the US Bible thumpers here, of course ~ many of whom are flat-earthers who deny evolution and see scientists as evil people who are constantly trying, and succeeding, in disproving their beliefs one by one.

Are you already having anal sex?

And how do you deny hundreds of years of advancement in all fields of science?

The people of the modern world have evolved past all of that nonsense.

Notice that I said their rules, not any gods rules.

What do you like about McDonald's?

Those rules that you are trying to reinstate are rules made by men who didn't know anything about the earth, nor anything about all of the other inhabitants of it. How can you not see that?