Here is a link  to an excellent article on same-sex marriage. The argument it makes against it is very much the kind of argument advanced by The Iona Institute but it is very well put and well worth reading.
Here is the key passage:
“Redefining (or “undefining”) marriage so that the state comes to embrace same-sex relationships as equally beneficial would deprive a class of children of their birthright to be raised by their natural mother and father. It would advance the notion that children do not need both a mother and a father, let alone their own mother and father. Same-sex parenting would send the message that parenting is not naturally gendered. There would be nothing known as “mothering” or “fathering,” only unisex “parenting.” Both culture and law would be unable to stress the important role that fathers play in their children’s lives without giving offense to “alternative” families in which there simply is no father. This would have disastrous effects for our nation’s children. Even the left-leaning research organization Child Trends affirms the importance of married mothers and fathers to child well-being. In a research brief summing up the scholarly consensus, they write:
‘Research clearly demonstrates that family structure matters for children, and the family structure that helps the most is a family headed by two biological parents in a low-conflict marriage. Children in single-parent families, children born to unmarried mothers, and children in stepfamilies or cohabiting relationships face higher risks of poor outcomes. . . . There is thus value for children in promoting strong, stable marriages between biological parents.’
Regardless of the name given to it, the state’s promotion of any pairing of adults as the functional equivalent of marriage would eliminate in law and weaken in culture the ideal that children should be raised by their married, natural parents. The public meaning and purpose of marriage would be ended; marriage would be redefined as merely a private relationship of consenting adults, and parenthood as merely a legal status applied to those who choose to take responsibility for a child. The social function marriage plays in society — providing children with their natural mother and father — would be lost.”