In The Other Wes Moore, why did Mary once refuse to let Bernard, Wes's father, see Wes as a baby?

Bernard got Mary pregnant with Wes, but he abandoned her before Wes was born. Bernard is an abusive alcoholic with no education who finds it impossible to hold down a steady job. It's little wonder, then, that Mary doesn't want him in her son's life.
When Wes was eight months old, Bernard unexpectedly showed up on her doorstep, banging loudly at the door and demanding to see his son. But Mary stood firm; she wouldn't let Bernard in. Bernard was drunk, as usual, and Mary clearly felt that he represented a danger to young Wes. She may also have been thinking about the long term, that if she allowed Bernard across the threshold he'd start trying to insinuate his way back into her life, with possibly damaging consequences for the stability of Wes's upbringing.