But, if the end result is that a man and a woman doing the exact same job with the same perf ratings at Google are paid differently, because the woman happened to be paid less at her previous job - is that fair?
There is a reason that NYC just outlawed companies asking for salary history.
I just find it dishonest to frame the question as about men and women, as you did just now, if your concern is that people who negotiate a worse first salary are paid less. You're at best co-opting an unrelated issue.
I would say that in answer to your question, it's not fair, just as it wasn't when it happened to two men, but I'm not sure it's something we should change.
I'm also going to say that you should stop framing it as a gender equality issue when it's not -- it's a capitalism implementation issue.
I've noticed that many of these complaints about gendered things are arguments for a particular social arrangement (eg, no consequence to a break in career) and not arguments against sexism, as such.
I don't think we shoukd be using equality laws as a bludgeon to effect changes in society outside of things like sexism or racism: that's what causes Trumpian backlashes. Most people are onboard woth fighting sexism; it becomes a mixed bag when talking about proposals like "taking years off should have no result on earnings".