Historian David Barton said the Constitution bans gay marriage and other speakers praised past efforts by Utah legislators and voters to prohibit gay marriage in the state.
Rep. LaVar Christensen, R-Draper, the author of Amendment 3, said the Supreme Court ruling was "mostly procedural, not as much substantive."
"The joyous news is that Utah stands unaffected," Christensen said. "America needs Utah like never before."
Frank Mylar, a marriage law attorney and former Utah assistant attorney general, said he saw positives in the ruling despite the court striking down a key DOMA provision preventing gay marriage.
"The very important provision, perhaps equally important provision, that prevented one state from validating marriage and moving to another state like Utah and trying to enforce that upon Utah jurisdiction, that provision was not invalidated," Mylar said.
Twelve states most of them on the East Coast and Washington, D.C., have passed laws legalizing same-sex marriage. Utah neighbors Colorado and Nevada allow civil unions between gay couples.
Proposition 8, a California initiative that was backed by the LDS Church and which banned gay marriage in that state, was partially struck down in a second ruling by the high court Wednesday. The Supreme Court left alone an appellate court's ruling that Proposition 8 was unconstitutional, opening the way for gay marriage to return to California and making the state the 13th to allow it.
"Same-gender marriage defines each of us as interchangeable, exchangeable units, and that there's no difference between men and women," said the event's co-chairwoman Mary Summerhays.
About a dozen protesters stood outside the Expo entrance in support of same-sex marriage.