His step-daughter is 40 and separated from her husband. Like her mother before her, the younger woman believes oral sex to be dirty and sinful. Because of this, both women refuse to satisfy their man in this manner, therefore causing the breakup of the younger woman's marriage.
The older man knows this because his step-daughter confides in him when he stops by to find out what went wrong in her 20-year marriage. When the step-daughter says she misses her husband very much and would do anything to get him back, the caring stepfather shows his dead wife's daughter that mom isn't always right.