It seems Mike Huckabee is looking for a way to explain exactly what marriage means and why it should not be given to same-sex couples. Same-sex marriage wouldn't necessarily "redefine" marriage, unless the question of conception rights for same-sex couples was decided separately, and then only if same-sex conception was prohibited. Same-sex marriage would give all the rights of marriage, including the right to have or attempt to have biological offspring, to same-sex couples. As long as same-sex conception is allowed (and it is, in every state except Missouri), then it changes marriage's meaning more to deny marriage to same-sex couples than it does to allow it. And it would change marriage's meaning just as much to deny a married same-sex couple the right to attempt to create biologically related children together, which we would do if we were to enact a law against human genetic engineering while still allowing same-sex marriage.
We need to preserve marriage's conception rights, by affirming that every marriage has the right to conceive offspring using the couple's own gametes. And we need to prevent genetic engineering, by prohibiting all methods of creating a person except joining two people's (unengineered) gametes, which means fertilizing a woman's egg with a man's sperm. Same-sex conception requires genetic engineering.
Marriage equals conception rights. The question is, should we allow same-sex conception or not? Does a person have the same right to conceive with someone of either sex, or only with someone - publicly, legally - of the opposite public, legal sex? That is what we need to ask before allowing same-sex marriage, or else we are changing marriage's meaning by stripping it of the guaranteed right to conceive children with the couples own genes.