My 2 x g.grandfather was illegitimate and when I first bought what I thought was his marriage cert, (it turned out to be the correct certificate), I discounted it because there was a totally different name for his father and at that time I hadn't found out that he was illegitimate.
Since searching on FreeREG, I see that he was illegitimate and that is what is listed in the Parish Register. However, as my 2 x g.grandfather was born in a very small village in Lincolnshire where this man also lived, I am fairly certain he must have been his father, or like other illegitimate ancestors I have he would have just made up a name, giving the "supposed" father the same surname as himself.
Unlike Andrea's experience, I know my 2 x g.grandfather didn't have a stepfather, his mother died in a workhouse still single, when 2 x g.grandfather was only 16.
Lizzie