Reid, a Democrat from Nevada, is the highest ranking elected official who is a member of The Church of Jesus Christ of Latter-day Saints. He previously has not commented on the flood of Mormon money and volunteers who helped propel Proposition 8 to victory in November.
But three organizers of the past weekend's National Equality March said Reid brought up the topic during a conversation in his office.
"He said that he thought it was a waste of church resources and good will," said Derek Washington, a Nevadan who worked as the outreach director for the march. "He said he didn't think it was appropriate."
Reid spokesman Jon Summers would not discuss the private meeting, but he didn't deny the conversation took place.
"While Senator Reid agrees with his church that marriage is between a man and a woman," Summers said, "he also believes that the resources that went into the Proposition 8 effort could have been put to better use."