I believe the specific law is from 1977, but I agree with your point in general, that service shouldn’t discriminate based on gender.

From my knowledge this rule was supposedly active for decades before Germany discontinued conscription and nobody really followed it then. Why there is a need to reactivate a law that will not be followed by the vast majority and that has no enforcement mechanisms associated with it really beats me. It’s probably a mix of incompetence and “we’ve always been doing things like this”.