"One of those, under normal circumstances, naturally results in the continuation of the species through procreation, and of society through a healthy family life. The other does not fulfill either of those roles, and cannot, under any circumstance, fullfill the most important one.
Marriage isn't about legitimacy, or about legal benefits, or about property rights - it's about creating and raising a family."
If a gay couple marry, they have produced a family. Not all families have children. Are you claiming that childless marriages are not real marriages and families?