It probably could be challenged under the German constitution, but nobody knows if that would be successful. The draft for men is set up in the constitution, but there is also an explicit equality for men and women in there. In the past any challenge would almost certainly have been denied, but it's a different time now.

In practice, this draft is not a real draft yet. Nobody is actually drafted, so there are almost no practical consequences. If there was an actual draft, I'd expect to see a challenge to this.