Enshrined in Law since 1930: Only men can be slaves

Via Marc A. Been looking for more "cold hard evidence"? It's right here. Excerpt:

"C29 Forced Labour Convention, 1930
Convention concerning Forced or Compulsory Labour (Note: Date of coming into force: 01:05:1932.)
...
Date of adoption:28:06:1930
...
Article 1

1. Each Member of the International Labour Organisation which ratifies this Convention undertakes to suppress the use of forced or compulsory labour in all its forms within the shortest possible period.

2. With a view to this complete suppression, recourse to forced or compulsory labour may be had, during the transitional period, for public purposes only and as an exceptional measure, subject to the conditions and guarantees hereinafter provided.
...
Article 11

1. Only adult able-bodied males who are of an apparent age of not less than 18 and not more than 45 years may be called upon for forced or compulsory labour.

...
2. Nevertheless, for the purposes of this Convention, the term forced or compulsory labour shall not include--

(a) any work or service exacted in virtue of compulsory military service laws for work of a purely military character;"

Note that as Marc A. pointed out in his note to MANN, this convention specifically excludes military service from its scope. In other words, this document is not the smoking gun with regard to the modern international standard for forced military service, but is the smoking gun regarding *any* forced service exacted by an authority from the population-- and it specifically says only able-bodies males between 18 and 45 are subject to it.