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.
I made the assumption that all else was equal. A family would be the scale tipper. In the case you mentioned it does not sound like the married guy is anything like the single man and the choice was a no brainer.