Democrats have majorities in both houses of the legislature the Assembly speaker and Senate president say they think only a civil-unions bill would win enough votes to pass.
New Jersey's legislature passed a bill recognising civilunions between gay couples, which gives same-sex partners the same rights and benefits as married people in the state.
Advocates had expected that Colorado would become the first state that has banned gay marriage to pass a bill allowing civilunions, which grant certain legal rights to same-sex couples.