The Maryland Senate on Thursday approved a gay marriage bill.

After a lengthy debate, one Republican, Senator Allan Kittleman, crossed the aisle to join 24 Democrats in voting for the bill.

Senator Richard S. Madaleno Jr., a Democrat and the chamber's only openly gay member, argued that giving gay and lesbian couples the benefits of marriage is a matter of fairness.

“It provides full equality under the law for thousands of same-gender couples in our state, couples like Mark and myself,” he said, referring to his partner of more than a decade, with whom he's raising two children.

Opponents of the bill suggested that legalizing gay marriage would open the door for state recognition of polygamous or incestuous relationships.

“Here's my question: Where does it stop?” asked Senator C. Anthony Muse, a Democrat and a pastor.

Senator Bryan W. Simonaire, an Anne Arundel Republican, warned that “young, impressionable students” would be taught “the homosexual worldview” if gay marriage is legalized, a claim used to repeal laws in California and Maine.

The bill now moves to the House, considered the more liberal chamber on social issues. However, supporters on Thursday acknowledged they have yet to lock down a majority. A hearing is scheduled for Friday.

Governor Martin O'Malley has pledged to sign the bill into law, which would make Maryland the sixth state to legalize the institution.

Opponents of marriage equality have promised to mount a campaign to repeal the law, if approved by lawmakers.