... is that the so-called "gender pay gap" is actually due to life decisions, not rampant sexism?
I'd say it's much more likely it's due to life decisions, priorities, and preferences instead of rampant, systematic, and explicit sexism.
Mark Perry writes about this often at the AEI blog. Let's take the 20% number at face value. That means that if you pick two identical employees (same job, same tenure, same experience, same skills, same performance ranking), and one is male and one is female. The 20% story says the man, on average, gets 20% more income than the woman. In some cases the gap will be zero, in other cases it would be much higher.
For this to be true for the entire country, it must be rampant. Cases like that must be all around us. Name one. Seriously, name one. Do you know of any actual cases with actual people at your place of work? I don't because I don't know how much people get paid, so I can't confirm or deny this. I do know one of my previous employers did have explicit pay scales and ranking system. I knew how everyone was ranked and I knew the pay scale. The company made it very transparent what your raise ought to be based on your current pay and ranking. There's no way women were getting smaller raises than men. It is certainly unbelievable there were separate rate ranges for men and women. And I saw the ranking distribution. I don't believe it was grossly unfair to the women.
Now, I didn't study the results. It's entirely possible there was subtle sexism in the process. Maybe we as a group tended to rank women lower than men. I don't know and I can't rule it out. But I'd be really surprised if something like a 20% lower ranking (and it would have had to be much more than that) wouldn't stand out.
There are lots of other examples which make it hard to believe the 20% story. Government workers have strict civil service procedures to grade and pay people. That's millions of people. The process is so bureaucratic it's very hard to believe there's a secret bias as big as 20%. Again, it would have to creep in by not giving women raises or good performance reviews, which could happen. But you're not going to solve that by just declaring women must get equal pay, you have to dig deeper.
That's evidence for a lack of bias. Evidence to explain the wage gap isn't too subtle. You can look at job categories with lots of men versus lots of women. For example, take health care. Nursing tends to attract women while surgical specialties attract men. Nurses also tends to get paid less than heart surgeons. Why people make these choices is beyond me. It could be sexism, women being told they can't be surgeons or some such. I could totally believe that exists. But again, you're not going to solve that by declaring that all heart surgeons get paid the same. And you're not going to close the wage cap by mandating all heart surgeons get paid the same.
So, I believe there is a substantial wage gap. It seems much more plausible to me that most of it can be accounted for by life choices. I am willing to believe that sexism and social pressure influences people on some of those life choices. I believe the problem is too deep to be solved by just declaring all people in a certain job category must be paid the same.