Does same-sex marriage threaten the institution of marriage? That has long been a staple charge by leaders of campaigns for federal or state laws to prohibit homosexual marriage.

Authors of a recent op-ed page article in The Wall Street Journal disagree. Darren R. Spedale, a New York investment banker, and William N. Eskridge Jr. take issue with Bill Frist, the outgoing Senate majority leader, who failed last summer to get the Senate to approve a prohibitive constitutional amendment.

They recall that Sen. Frist stated that “years of de facto same-sex marriage in Scandinavia has further weakened marriage” and that similar claims had been made by Sens. John Cornyn, James Inhofe, Sam Brownback and Rick Santorum.

The authors said there is no evidence that allowing same-sex couples to marry weakens the institution: “On the contrary, the numbers indicate the opposite.”

They cited records a decade after Denmark, Norway and Sweden passed partnership laws: Heterosexual marriage rates had risen 10.7 percent in Denmark, 12.7 percent in Norway, and 28.8 percent in Sweden. And divorce rates among heterosexual couples dropped 13.9 percent in Denmark, 6 percent in Norway and 13.7 percent in Sweden, and remain lower than before the same-sex partnerships were legalized.

Cause and effect, of course, is hard to prove, as the authors acknowledge. Logicians call it a logical fallacy to assume that if one event happens after another, then the first must be the cause of the second. So statistics can lie.

But the senators also violate this ancient rule of logic and don’t even have statistics to make their case.

Messrs. Spedale and Eskridge go on to dispute the “slippery slope” argument – that same-sex marriage would start a dangerous movement toward legal recognition of socially unacceptable relationships. They note that this hasn’t happened in Scandinavia. After 17 years, there are no calls there for recognizing polygamy, incestual marriage or marriage to animals.

Still, same-sex marriage remains as hot-button political issue. Gay marriage bans passed in seven states on Nov. 7, although Arizona defeated a ban proposal. In Massachusetts, the only state that permits gay marriage, outgoing Gov. Mitt Romney, now a Republican presidential hopeful, threatens legal action if the legislature doesn’t vote to put the issue on the 2008 ballot.

The authors wisely advise looking to no-fault divorce, prenuptial agreements and legal recognition of heterosexual cohabitation as the real culprits of weakened marriage: “As the evidence indicates, societies where gay couples have the rights of marriage seem to be doing just fine.”