Supporters of same-sex marriage have long argued that their foes were irrational because allowing gay and lesbian couples to wed had no impact on straight couples. Thanks to research published June 11 in the Public Library of Science, supporters of same-sex marriage now have empirical proof to back up that claim.

"[A] deleterious effect on rates of state rates of opposite sex marriage has been argued to be a motivating factor for both the withholding and the elimination of existing rights of same sex couples to marry by policy makers -- including presiding justices of current litigation over same sex couples rights to legally marry," Alexis Dinno and Chelsea Whitney of Portland State University wrote in their study.

Conservatives have argued that same-sex marriage detracts from the rights of "traditionally" married couples. Former Republican presidential candidate Rick Santorum, for instance, said in April that same-sex marriage discourages marriage between a man and a woman.

So the researchers set out to find whether rates of opposite-sex marriage changed as a result of same-sex marriage or same-sex civil union laws.

The researchers examined the thirteen states and the District of Columbia where same-sex marriage or same-sex civil union laws were implemented before 2009. By statistically analyzing the marriage rates from 1988 to 2009, they found the legalization of same-sex marriages or civil unions had no effect on the rates of opposite-sex marriages.

"We conclude that there is no relationship between implementation of same sex marriage or strong or weak same sex union laws and rates of opposite sex marriage," Dinno and Whitney wrote.

[Same sex marriage photo via Shutterstock]