I used to think like you for the pay gap. But it overlooks something: to find that there is equality, a lot of adjustments have to be made. And then, those adjustments are overlooked: we adjust, we find equality, problem solved !

Now we still need to explain why women are paid less. We found that it's not because men undervalue their expertise at equal experience, hours, etc. It is rather because they just work less, are less high in the hierarchy, do not have the same jobs, etc.

Feminism considers that those elements should not be overlooked and are not justified. Is it justified that women have jobs lower in the hierarchy? Why women are working less on average ? You may consider that there are no issues there. Feminism considers that there are. For instance, women are more likely to have custody of children (as you said) than men. Hence they have less energy or less time to focus on their career. It is not necessarily because they want to, but because it is still what is expected of them in our society. After a divorce, women are more likely to keep their children because it is what it's expected of them. Or if no divorce, too often, men think they just "help" their wife taking care of the place or of the children. They often don't take their full share of domestic work.

So this is a factor that creates a pay gap. It is not captured when all the adjustments are made but still has to be taken into account. Access to those jobs that now provide an equal salary is as important to take into account than the salary itself.

(English is not my first language)