The point is that unlike things like race discrimination, when all other things are equal, there IS a real difference, no matter how small, in term of risk (and in practice, it's really not that small). I worked at a large company where most employees were women in the 20-35 yo rangeny for a while on HR project and saw some of the data. It was not pretty.
As a society though, we shouldn't let that be ground for discrimination. As you mentioned, not all women want or will have kids. Of those who do, many will have supportive significant others, and the difference in productivity will be negligible. Even if it wasn't, I don't want to live in a world where people have trouble getting a job because they might end up doing something a very significant portion of the population will do.
But it's still a fact that, given 2 exactly equivalent candidate, both in their mid twenties, one is a guy, one's a woman, there is a well known, significant short to medium term risk in the later, if only looking at it from a local maximum perspective. We as a society need to find a way to artificially make up for it.
My personal favorite solution is to give both men and women equal (mandatory?) parental leaves, and no difference if its an adoption, same sex partners, or anything like that. Yes, women have an actual medical need, yes it might not be quite representative of reality, but that equalizes the risk from an employer perspective. No difference between the man and the woman (bonus point, it will help even out parental contribution and responsabilities at home).
That still leaves the age discrimination, but considering ageism is often against older people, that might really just even things out a little.