Massachusetts lawmakers gave preliminary approval Thursday to a constitutional amendment that would ban gay marriage but allow civil unions.

The amendment, which would strip gay couples of their court-granted marriage rights, must still weather several additional votes and anticipated legislative maneuvering by opponents.

The earliest a ban could end up on a statewide ballot is November 2006, more than two years after same-sex couples can start getting married in Massachusetts.

Due to the elaborate constitutional-amendment process, the ban must be approved by the Legislature at least three more times this year — perhaps as soon as Thursday night — and then again during the 2005-06 legislative session. Then it would go before voters in 2006.