(08-04) 19:09 PDT – If voters approve a November ballot measure banning same-sex marriages in California, thousands of gay and lesbian weddings conducted since the state Supreme Court legalized the unions on May 15 will probably remain valid, Attorney General Jerry Brown said Monday.

The potential effect of Proposition 8 on existing same-sex marriages is already being debated among legal scholars and opposing sides in the Nov. 4 ballot measure campaign. Brown’s position is significant because his office will represent the state in lawsuits over Prop. 8’s validity and meaning if it passes.

The measure would amend the state Constitution to declare that “only marriage between a man and a woman is valid or recognized in California.” It would overturn the court’s ruling that the previous ban on same-sex marriage - established by statutes rather than a constitutional amendment - discriminated on the basis of sexual orientation and violated the right to marry one’s chosen partner.

The measure does not state explicitly that it would nullify same-sex marriages performed before Nov. 4. But in their ballot arguments, supporters of Prop. 8 declare it would invalidate all such marriages “regardless of when or where performed” - an interpretation that would apply to existing as well as future marriages.

Courts that interpret ballot measures sometimes refer to such statements as evidence of what the voters knew and intended.

Brown’s office said in a court filing Monday that he believes the ballot measure would not invalidate existing marriages.

“I believe that marriages that have been entered into subsequent to the (May 15) Supreme Court opinion will be recognized by the California Supreme Court,” Brown told The Chronicle. Noting that Prop. 8 is silent about retroactivity, he said, “I would think the court, in looking at the underlying equities, would most probably conclude that upholding the marriages performed in that interval (before the election) would be a just result.”

Brown’s legal filing was among a flurry of competing arguments submitted Monday to a Sacramento County Superior Court judge, who is scheduled to hold a hearing Thursday on challenges to the ballot materials that will be sent to California’s 16 million registered voters.

One issue is Brown’s title for Prop. 8, which declares that the measure “eliminates the right of same-sex couples to marry.” The Yes on 8 campaign said in a lawsuit last week that the title was “inherently argumentative and highly likely to create prejudice” against the measure. The campaign asked the judge to order a different title, such as “Limit on Marriage,” which was used during signature-gathering before the court ruling.

Brown defended his ballot language as neutral and accurate Monday, while opponents of Prop. 8 pressed their challenge to a section of the Yes on 8 argument. That argument declares that teachers will be required to tell students as early as kindergarten that “gay marriage is OK” unless the ballot measure passes, a claim that the No on 8 campaign calls baseless.

In another development Monday, the California Catholic Conference, representing Catholic bishops, endorsed Prop. 8, declaring that “same-sex unions are not the same as opposite-sex unions” and that only a relationship that can produce children should be called marriage.

E-mail Bob Egelko at begelko@sfchronicle.com.