All animals are equal, but some are more equal than others.

Registration is compulsory, but actual military service is voluntary (for now).

In other words, it's functionally the same as Selective Service forms in the US.

One, I'm not sure what American founding ideals have to do with Germany.

Two, Germany, like most countries and frankly human populations, has a male surplus in its fighting-age population [1]. This is why, historically, large socities tended to wage war with men first. (Even those that e.g. held elite units in reserve, which undermines the usual biological argument.)

[1] https://en.wikipedia.org/wiki/Demographics_of_Germany#/media...