From a Beliefnet interview:
And the same thing would be true of marriage. Marriage has historically, as long as there’s been human history, meant a man and a woman in a relationship for life. Once we change that definition, then where does it go from there?
Christians and modern Jews do not approve of polygamy, but surely anyone who believes in the Bible has to acknowledge that it attests to the widespread existence of marriage between a man and multiple women. (Nor is Huckabee just saying "a man and a woman" as a slip for "heterosexual"; immediately after this, he goes on to distinguish "a man and three women.")
Now of course we see polygamy in plenty of cultures in recent human history as well — Islamic cultures, some American Indian cultures, and many more. And of course we have lots of historical evidence of men and women in a relationship that is not for life; consider ancient Rome, or for that matter the growing tolerance for divorce in Western Christian cultures over what is now centuries (remember Henry VIII?). But it's striking that here Huckabee is forgetting what is described in the Bible itself.
I should stress that none of this responds to the traditionalist case built on longstanding American or Christian traditions of heterosexual monogamy, or for that matter to a traditionalist case built on longstanding broad traditions of not treating homosexual unions as marriages. But Huckabee seems to be deliberately trying to make an appeal to supposedly universal (at least nearly universal) traditions that go beyond just rejection of same-sex marriage. And that appeal is just factually unfounded, as his own religious histories and his own profession (as minister) should teach him.
UPDATE: I had thought I'd made this clear in the original post, but let me repeat it: I'm objecting to Huckabee's "historical" claims, and saying they're inconsistent with the Bible's own account of history. I am not responding to Huckabee's moral claims; I am criticizing his attempt to buttress his moral claims with what strike me as factually unsound (and Biblically contradicted) assertions about what has been the case throughout "human history."
FURTHER UPDATE: My colleague Stephen Bainbridge (who's considerably more conservative on social issues than I am) agrees.