What percentage of 18-25 year old women are housewives staying at home and raising kids? And if their husbands are drafted, how to they make ends meet? Go on welfare?
In any case, you can always have (initially at least) a deferment to both sexes with young children. Or better yet, get rid of the Selective Service entirely. There are plenty of good arguments on this thread for that.
The very idea of females being required to register comes from the feminists, and they want all females to have their beliefs set aside, and forced into a new "cultural construct."
With the precedent already set (in the minds of military planners) by young women who are presently willing to leave their children behind and go out and spend months in Humvees with men to whom they are not married, it points to difficulty getting deferments in the future. We have already crossed the threshold of "Because some women WANT TO, all women MUST" (again in the Pentagon think tanks)
Millions of American women, not just a few, still have firm convictions on women daily having the hands-on training responsibility of their own children. There should be NO threat at all to this.