Utah Celebration of Marriage projected the images on a screen at a rally held Thursday in the Capitol Rotunda to show support for what the group describes as "traditional families."
Conservative activist Mary Summerhays showed photos of two men holding their son and two women with their baby daughter.
Summerhays told the crowd the children "will pay the prices of redefining marriage."
One of the families shown in the photos was Matthew Baraza and Tony Milner and their son. Baraza and Milner are plaintiffs in the lawsuit seeking to force Utah to recognize the same-sex marriages conducted in December and January.
On Saturday, Summerhays defended the use of the pictures.
"These photos were featured repeatedly in the Deseret News and Salt Lake Tribune beginning as early as January 10th, 2014," she wrote in an email, "and illustrate that a child will lose at least one biological relationship when it comes into conflict with a same-sex marriage."
Whether same-sex marriage is harmful to children has become a contentious point in the dispute over marriage in Utah.
On Sept. 29, the U.S. Supreme Court's nine justices are slated to discuss whether to take up a same-sex marriage case in its upcoming session. Among the seven lawsuits under consideration is Utah's landmark case of Kitchen v. Herbert, the first in the country in which a federal judge overturned a state ban on same-sex unions on the basis that it violates the U.S. Constitution.