The 30-second ad from Preserve Marriage Washington is running statewide on both broadcast and cable channels.
Referendum 74 asks voters to either approve or reject the state's gay marriage law that was passed by the Legislature and signed by Gov. Chris Gregoire earlier this year. That law is on hold pending next month's vote.
The ad says that because gay and lesbian couples already have all of the same legal rights of married couples under the state's current domestic partnership law, that R-74 should be rejected.
"You can oppose same-sex marriage and not be anti-gay," the ad says.
The ad, which opens with a picture of a rainbow flag, also warns of potential lawsuits or fines for those opposed to same-sex marriage if the referendum is approved.
Preserve Marriage spokesman Chip White pointed to a recent lawsuit settlement in Vermont between an inn and two New York women over an accusation it refused to host the couple's wedding reception. He cited cases from other states that have civil unions or domestic partnerships that have seen similar lawsuits.
"I don't think we need to limit ourselves to only looking at the states that have same-sex marriage to see what kind of lawsuits happen when marriage is redefined," he said.
But Andy Grow, a spokesman for Washington United for Marriage, said discrimination based on sexual orientation is already illegal under the state's anti-discrimination law, which was expanded in 2006 to cover gays and lesbians. And, he notes, no known lawsuits like White points to have occurred in Washington state under the state's current "everything but marriage" domestic partnership law that grants gay and lesbian couples all the state-granted rights and benefits that married couples have.
Washington United for Marriage, the campaign working to uphold the law, issued a statement saying the ad's sole intention was to mislead and scare voters.
"Simply put, our law hurts no one, and better protects all loving couples and their families," wrote campaign spokesman Zach Silk.
Same-sex marriage is currently legal in New York, Connecticut, Iowa, Massachusetts, New Hampshire, Vermont and Washington, D.C.
Maryland legalized gay marriage this year as well, but that state will also have a public vote this fall. In Maine, voters will decide on an initiative to approve same-sex marriage three years after a referendum overturned a law passed by the Maine Legislature. And in Minnesota, voters will decide whether or not to pass a constitutional amendment to ban gay marriage there.
The first television ad supporting gay marriage in Washington state ran during the Olympics in July, and was paid for by the Seattle-based Pride Foundation. That ad featured former Republican Sen. Cheryl Pflug, who voted for the gay marriage bill passed by the Legislature this year. Since then, Washington United for Marriage has run a series of ads in support of the measure.
Preserve Marriage, which pushed to get the referendum on the ballot, has raised more than $1.7 million in opposition to the law. Washington United for Marriage has raised more than $8.9 million.