From George Q. Flynn’s The Draft, 1940-1973:
In 1939 there were 1,404,000 marriages of those of all ages. In 1940 among the age cohort 18 to 29 there were just over 5 million marriages.
The Selective Service Act was enacted in September 1940.
There are actually some good reasons for having a military draft, though I’m not sure this is really one of them. There are some practical problems, particularly in an age of radical feminism and no-fault divorce.