The proposed initiative would also not allow civil unions (search) for same-sex couples. A separate, proposed amendment now pending before the Legislature would ban gay marriages but legalize Vermont-style civil unions.
"Governor Romney believes that voters should be given a straightforward amendment to decide the definition of marriage and not one that muddies the water by creating civil unions that would be equivalent of marriage in all respects but name," said Romney's spokesman, Eric Fehrnstrom.
Opponents of gay marriage plan to annouce the new citizen initiative later Thursday.
The state's highest court ruled in November 2003 that the state constitution guaranteed same-sex couples the right to marry. The nation's first state-sanctioned, same-sex weddings began taking place May 17, 2004, and since then, thousands of couples have tied the knot.
However, the Legislature last March passed a proposed constitutional amendment that would ban the marriages but allow for civil unions. That proposed amendment must be passed in identical form again this year in order to appear on a state ballot in November 2006.
The latest initiative could not appear on a state ballot until November 2008 at the earliest.