It may well be that there is substantial harm? Clearly infertile couples and elderly couples can and do adopt and become legal guardians of children and form traditional nuclear families all the time. We also do know that single mothers and children from divorced families are at a substantial disadvantage over children raised in traditional two parent, husband and wife nuclear families and these children are a substantial burden on their communities.
Gay adoption is pretty recent and the data isn't available yet for States to make reasonable conclusions one way or another. States certainly can decide with limited data to make gay marriage legal but I don't see those States that make a choice to promote traditional marriage with good data to back up that choice as unconstitutional.