That would be the Civil Rights Act of 1964 which makes it illegal to discriminate on the basis of race, color, religion, sex, or national origin.

That's the one. It seems like an obviously good idea. Are all seemingly good ideas actually good ideas?

But when through seeming coincidence some 96% of tech jobs are held by men I'd say there's something larger afoot.

Through what seeming coincidence are 100% of NFL player jobs held by men? Or 80% of health-care jobs held by women?

My theory is sexism. It provides a testable hypothesis that has been proven multiple times. It makes predictions on other effects we'd see. Those have also been proven.

Sexism is (a) not mutually exclusive with group differences and (b) is, in many of its claimed forms, wildly inconsistent with many observable facts. As an exercise, I suggest you try making a list of such facts. You might start by listing job categories dominated by women. (I'll get you going: health care, day care, education, publishing.) As a follow-up, try listing areas where men are objectively worse-off than women. (Maybe start with: homelessness, intellectual disability, autism, workplace fatalities, imprisonment, lifespan.)

If you scratch most supposedly definitive examples of unfairness caused by sexism, you'll find some variant of the Blank Slate hypothesis underneath. Once you admit the possibility that men and women might not be blank slates, many of the patterns that appear to be "sexist" suddenly make sense. For example, many people assume that women and men have the same innate aptitude and inclination for "tech" (broadly defined). From this point of view, any deviations from "equality" (i.e., proportional representation) must derive either from overt sexism or from an endemic sexist culture. But when you accept the possibility that men and women may differ in more than stature and genitalia, suddenly it's not so clear.

What's yours?

Due to a combination of genetic and non-genetic differences, different groups are systematically different, and have different aptitudes and inclinations. Group differences lead to a first-order effect: all NFL players are male because all qualified players are male; some majority of those inclined to health care are female. The preponderance of a particular group also leads to a group culture reinforcing group identity. This leads to a second-order effect: female health-care workers create a feminine culture, attracting further female workers (while repelling male workers). This latter effect is sometimes called "sexism", but it's an inevitable consequence of human nature. Efforts to fight it will (a) cause massive collateral damage while (b) not working. (Has the Civil Rights Act of 1964 brought racial and gender harmony? It seems to me that in many ways things are worse than ever.)

I'd guess we actually agree on many things, and in particular we want many of the same outcomes—or, at least, I want a subset of the outcomes you want. Let's say I want O and you want O + O'. Included in O is people of all races, colors, religions, genders, and national origins living prosperous, fulfilled lives free (as much as possible) of conflict and strife. Included in O' is the Civil Rights Act of 1964.

Often (as with the Civil Rights Act) the intent of policies in O' is to achieve outcomes in O, but I believe that not only does O' not achieve O, it frequently achieves not-O. Believers in O' would rather wreck college athletics with Title IX, or wreck the black community with welfare, than admit that O' doesn't achieve its stated goals. That would lead to their being accused of "sexism" or "racism", and O'-believers are as eager to avoid such accusations as they are quick to deploy them against their enemies. (Whereas I don't care, because I see them for the Orwellian terms of abuse they are. Just call me a "crimethinker" and be done with it.)