I think was this was meant to mean was: Any citizen has the right to join their local/state militia, and keep their service rifle and uniform with them at home.
Obviously this should probably be expanded to include women generally, but their 2nd Amendment rights are not predicated on their membership in the militia.
if able-bodied or militia is defined by the law does that mean they can include and exclude whoever they wish by redefining it?
Seems sensible. But the court is saying that if you look at what laws were passed / in force at the time, and what courts said about them around that time, you'll see that to contemporaries of ratification the second amendment meant a right to carry arms, not just to be in a militia, or whatever. That too seems sensible. Now what?
While the first part of the sentence says why should care, the explicit right is owned by the people.
What people don't often consider is that one can infer that rights might be infringed. The second amendment just says, "We can't fringe this right".
...however in the absence of militias in the US today, it makes sense that even if that requirement was initially envisioned (which is very debatable), it's no longer reasonable.
It's historically interesting to note that different versions of the amendment were ratified, both including and not including that last comma.