Elsewhere, couples held ceremonies on beachfronts and in backyards and living rooms.

“We kind of said Proposition 8 was like our version of getting knocked up,” said Benjamin Pither, 28, who married his high school sweetheart, Joseph Greaves, on Sunday at Mr. Greaves’s parents’ house in Santa Rosa. “We both liked the idea of marriage, but we wanted to do it in our own time. But when it looked like Proposition 8 might pass, we realized that we would regret it if we didn’t take the opportunity.”

Some couples traveled from afar to make Monday the big day. Allison and Rose, a lesbian couple from Tampa, Fla., said they had come to San Francisco to marry on the advice of friends who suspect that Florida will pass its own constitutional ban on Tuesday on same-sex marriage. The couple, who said they might relocate if Florida passed its ban, did not want their last names used because of fears that they would face discrimination at home.

“It isn’t like San Francisco,” Rose said.

While defeat of the California ballot measure would probably quell debate — at least for a time — over allowing same-sex unions in the state, it is expected that a victory would lead to a second round of legal wrangling over the validity of the thousands of marriages performed since June, when a State Supreme Court decision legalizing same-sex marriages took effect.

California’s attorney general, Jerry Brown, has said he believes that the marriages will remain valid, but Geoff Kors, the executive director of Equality California, a gay rights group that opposes Proposition 8, said he expected challenges.

“It wouldn’t surprise me that people trying to eliminate constitutional rights would try to annul or divorce people that are married,” said Mr. Kors, who expressed optimism that the ballot measure would fail.

Supporters of the ban say no rights would be infringed by its passage but suggest that the California Supreme Court will “have to deal with the mess that it made” by allowing the marriages in the first place, said Sonja Eddings Brown, a spokeswoman for Protect Marriage, the leading group behind Proposition 8.