Yet, within a decade social scientists began documenting very distinct harm to children, he said. "There is now a large body of irrefutable evidence of the serious, harmful effects for children of divorce that have been documented."

While the impact is "temporary for two-thirds, it is lifelong for about one-third," he said. Making same-sex marriage legal "will harm you and your family the same way polygamous marriage to 14 year olds will harm you. ... It will transform the meaning, expectations and practices of marriage as a social institution and affects everyone who has a stake in marriage."

Legalizing such relationships would affect the functioning of the entire legal system, he said, "from taxes to torts, from wills to medical treatment. The laws will change, and we'll reconceptualize our understanding that the union of two men or two women is equally important."

In doing so, taxpayers will incur "huge social costs," just as they do now when marriages fail. He cited a recent study by a business professor at the Institute of American Values putting taxpayer cost of marital breakdown and nonmarital childbearing at $112 billion per year.

"That's $70 billion in federal costs, $42 billion in state costs, and it amounts to over $1 trillion per decade. If you think fighting the war in Iraq is expensive, we've been paying those costs in this country for the past 30 years."

Estimates show those figures translate into about $4,500 per year, per family, in taxpayer dollars. "If your tax burden is high now, wait until those (same-sex) marriages fall apart and the state has to care for" divorced spouses and children of those broken unions, he said.

Traditional marriage "contributes much more to society than any other form of adult intimate relationship," and is the bedrock of "society's cultural infrastructure." It is the "instrument of the most important moral transformation of individuals," who enjoy the "most healthy, satisfying and socially beneficial sexual relations."

"Gay sex differs in critical ways," from that between husband and wife, he said, beyond the lack of offspring. "The major transmission method for the AIDS virus is through sex between men in every area of the world other than sub-Saharan Africa."

Wardle blasted the California Supreme Court's decision earlier this year legalizing gay marriage as "judicial legislation" that "weakened the most basic institution of society." The ruling was "based on assumptions that same-sex marriage contributes as much to society as gay marriage," and that notion is "not without consequences. They simply assumed the absence of harm and closed their eyes to contrary evidence. In fact, they refused to even examine it."