This is has been brought up a few times, but is largely a non-sequitur. First, if WWIII broke out, I doubt women would be exempt from a draft (e.g. see Israel where service is mandatory for women). Second, this is comparing exceptional circumstances where there is, presumably, an existential threat to the state vs. a situation that can reasonably be expected to occur in a person's lifetime (pregnancy).

The draft is also a temporary measure. As with other wartime restrictions of civil liberties, like curfews or rationing, it would not be tolerated if continued outside the context of a war.