With a landmark vote on same-sex marriage coming tomorrow, two prominent conservatives defended their stance against gay unions yesterday at a packed forum at Harvard Law School (HLS).

Tony Perkins, president of the Family Research Council (FRC), a conservative lobbying group, and Hadley P. Arkes, Ney professor of American institutes at Amherst College, told a standing-room-only crowd in Austin Hall that allowing gay marriages would weaken social institutions.

Last November, the Massachusetts Supreme Judicial Court (SJC) ruled that same-sex marriages are legal in the state. It has since decided that civil unions for same-sex couples would not be an acceptable alternative to full marriage status.

Tomorrow, the Massachusetts State Legislature will consider a state constitutional amendment banning same-sex marriages in the commonwealth.

“The courts are saying to the legislature and to the public, ‘We are going to impose this policy upon you,’” Perkins said. “The courts [are] usurping the legislative process where public policy is shaped by public debate.”

Perkins and Arkes also contended that there was “no historical evidence of discrimination” against homosexuals, and comparisons to African-Americans as a persecuted minority were inaccurate.

“The homosexual community is hijacking the bus and forcing African Americans to get in the back of the bus,” Perkins said, later adding that homosexuals “are better off economically and more educated than heterosexual couples.”

Arkes said the debate is complicated because it hinges on the term “sexual orientation,” which he said is “broad enough to encompass pedophilia, sadomasochistic sex, even bestiality.”

The audience appeared to be split evenly on the issue of marriage rights for same-sex couples.

Several local residents voiced their opposition to the SJC’s ruling.

“I have some deep resentment about the decision,” said Mildred Jefferson, the first female African-American graduate of Harvard Medical School. “It casts a shadow over fairness and justice.”

Jefferson received a round of applause after being introduced by Arkes.

Others in the audience were more critical in their questions.

“Would you consider verbal and physical abuse a form of harassment?” Sara Lam, a student at the Graduate School of Education, asked the panel.

“That happens to a lot of people. I have a mother-in-law,” Perkins replied.