"We ask the government to fix the problems we already have, not create new ones," said the Rev. Michael DeAscanis, a Roman Catholic priest in St. Agnes and St. William of York Parish in Baltimore.
The crowd roared with approval and broke into a chant directed at state lawmakers: "Do your job! Do your job!"
At another point, protesters yelled "one man and one woman" over and over again, stressing their support for traditional marriage.
The rally was intended to set a defiant tone before a Senate Judicial Proceedings Committee hearing scheduled Tuesday on O'Malley's bill to legalize gay unions. The governor is scheduled to pitch the legislation in person, a rare appearance before a legislative committee.
A same-sex marriage bill passed the Senate last year, but was pulled from the floor of the House of Delegates when Democratic leaders determined they were a few votes short of passage. That chamber never voted on the measure.
Over the summer, O'Malley pledged to include a similar bill in his legislative package for the 2012 session, and put the full force of his office behind passing it. If he succeeds, most expect that it will be petitioned to referendum and voters will decide on the November ballot.
On Monday, protesters latched on to remarks made last week by Maryland first lady Catherine Curran O'Malley, who called some delegates "cowards" for preventing the measure from passing last year. She later said she regretted her choice of words.
One man, who declined to give his name, held up a sign that read "Katie O'Malley, Only 'Cowards' endorse the immorality of LIBERALS." Another held a handwritten sign that said "Mrs. O'Malley, I'm not a coward." Several Republican lawmakers wore yellow buttons with the phrase "Proud to be a coward. Defend marriage."
Robin Robertson, 54, of Damascus said that he came to the evening rally because he feels "bullied" by "the gay agenda." A Catholic with five children, he said: "I feel like I'm having to accept something that I don't believe in."