You ask a good, but a culturally sensitive, question. Your question can be answered from several different angles. There is the medical/psychological one and a religious view. Let’s approach this purely from a medical/psychological standpoint because as a physician, I believe that understanding a child’s needs should come from sound research.
Here’s what we know: Children have emotional needs that must be met by fathers and mothers.
Research shows (I know this because I’m currently researching father effects on children) that fathers give very different things to children than mothers do. Both parents are equally important, but the effects a father has on a child are different from the effects a mother has on the same child.
For instance, we know that mothers read and speak differently to children than fathers generally do. Mothers tend to baby-talk toddlers and fathers will typically use more sophisticated language.
We believe this difference accounts for the fact that when fathers read to their six-month-old children, those children test higher on IQ tests when they are three years old. According to the research, the same is not true for mothers.
Additionally, we know that sons and daughters who grow up without fathers in the home (but who do have a mother) are at higher risk for getting kicked out of school, using drugs, being sexually active, being depressed and living with more anxiety. The list of benefits that both sons and daughters get from having fathers in the home is extensive and undeniable.
As a child advocate, I must educate parents on these studies because they are important to children’s health. My job is to coach parents to do what is medically and psychologically best for their children.
Since there is no debate about the fact that fathers and mothers fill equal but different important emotional needs for children, there is no doubt in my mind that all children need a father’s and mother’s influence in order to grow up to be as emotionally healthy as possible.
Similarly, I have worked with many single mothers who live in anguish trying to be a father and a mother. Once they realize the best they can do is be a great mother, they feel relief. Believing that they can be two people in one is overwhelming and makes everyone crazy. So I encourage them to recruit help from a good man they know–a father, brother, uncle, etc.–to mentor their child. This benefits both mom and the child.
Therefore, from a medical position, I always advocate that single mothers expose their children as frequently as possible to good father figures and that single fathers do the same with good mother figures.
In a lesbian couple, there are two parents, but again, no inherent father influence. Two women simply can’t bring a father’s influence to a child just as two gay fathers cannot be a mother to a child.