In my place of work, there are certain things you need for promotion, which require travel. I can't travel as I am breastfeeding ds, for example, so I can't apply for promotion yet.
Conferences are on weekends, seminars are in the evenings. This means it is hard to do the things where you network, and get your face/name known, ec, if you have small children and no support (like me as Dh works away, or when I was a single parent).
So, the way things are organised means that it is hard for people caring for small children (usually women) to progress in the same way as people without caring responsibilities (usually men). There is probably no reason why those two groups should fall along sex lines, apart from breastfeeding, but they do.
That is how I understand structural sexism anyway. It doesn't need to be like that, but sexism is built into the structures of organisations, and society more generally.