For me, it's the fact that my children are involved. Any man who came into my life would have to be kept totally apart from them, so that they didn't get attached to him and have him disappear two or three years later. They've already lost one male role model (albeit a bad one), I don't want them to have a series of men coming into their lives and disappearing again, leaving them with the impression that adult relationships between men and women are temporary.
Or he would have to marry me and be a permanent part of the children's life.
The problem is the middle stage - how do you get from keeping a man apart from your children so he doesn't damage them (emotionally) to a stage where he is an accepted and (hopefully) permanent part of your and their lives?
It is obviously possible - the fact that other people have done it proves that! But for me, I haven't stumbled across anyone in three years that seemed worth missing Coronation Street for a date with, let alone any more strenuous effort!