This past weekend I presented the workshop, "Homosexuality: Always God's Children" at the Credo retreat. We did not have time for the Q&A portion of the workshop, so the questions will be answered here during the next week or two.
Q: One of the arguments against gay marriage is that children need a mother and father. How do we explain to others the difference between gay parents and single parents?
A: Since our society is generally going in the direction of minimizing (or, ignoring) any difference between male and female, it's not surprising that many people see fathers and mothers as interchangeable. But personal experience likely tells us that we learned different things from Mom, and different things from Dad. Children who grew up with one parent often share stories about the things they were longing to know from the missing parent. There was something that couldn't be filled, no matter how much they were loved by the present parent.
So, just to affirm the beginning of your question, it is important that we recognize the difference between a father and a mother and the need for both.
But, how do we express the difference between being raised by a single mother or father and being raised by two mothers or two fathers? At first glance, it might even seem that the child is better off with two parents, regardless of gender.
The first point to make is that, generally speaking, single parents did not conceive their children with the intent of their children not knowing one of their biological parents. Single parents are the result of divorce, out-of-wedlock birth, or widowhood. Consequently, it was not in the parent's plan to keep their child(ren) away from Mom or Dad.
A same-sex couple is different. Through using artificial reproductive technology (IVF, sperm donation, egg donation, etc.), the child is deprived from the very beginning from knowing one of his biological parents. If two men are the parents, then the child is withheld from his biological mother (via egg donation and/or surrogacy). If two women are the parents, then the child is withheld from his biological father (via sperm donation). All children desire to know where they come from -- who they come from -- and this basic desire is denied to children who were conceived with the intention of being separated from one of the biological parents.
There are also differences between being raised by one parent and being raised by two parents of the same sex. Our family is the place where we learn about love, our identity, family, etc. Very different worldviews will be presented by a married couple, a single parent and a same-sex couple.
Various studies have been done to look at the effects of being raised by same-sex parents. The most comprehensive study was conducted by Mark Regnerus a couple of years ago. There is an easy-to-navigate website that explains the findings, available here. There are criticisms of every study on this topic conducted thus far, and it's true that we can't base everything on a study or survey. Still, there is interesting information available at the site.
Finally, it's important to underscore that the ideal environment for a child is a loving home between his married mother and father. We know that this is not the reality of many families today. Still, we cannot use the skyrocketing numbers of "alternative family structures" to affirm situations that are not best for children. It's a difficult topic because many families in less than ideal circumstances have been left in these situations due to tragic circumstances (divorce, death, break-ups, abuse, etc.). This answer is not to stand and judge families that do not have a present, married mother and father. Rather, it's a reminder of the importance of seeking the good for all children, despite the difficulties that may arise.