The employer was looking out for his own interests, which employee is going to work harder to keep the promotion, a single guy who can pick up stakes and move at any time or the man with a wife, two kids and a banker to support?
Exactly.
In my experience, you promote the grownup, regardless of marital status. We are assuming in this case that rather then being a blind hire this is a internal promotion where you know the parties involved.
Applying it to two young men in my own company currently I would go with the single. He is a grownup while, despite the wife and two kids of the other, the married man is still a child in many ways.
In other companies the situation has been reversed but, in the under thirty crowd, marriage is not very useful as a indicator of reliability. Maybe it was different before.