Casey's announcement, Martin said, gives elected officials "the ability to look at these issues and understand you can come to a conclusion after thinking about them."
The president of the Pennsylvania Family Institute, which opposes gay marriage, said Casey acted in response to pressure from special interests.
"There were many Pennsylvanians, Democrats and Republicans alike, who voted for Senator Casey because he respected many of the social conservative values of Pennsylvania — on the life issue as well as the marriage issue," president Michael Geer said. "I think many supporters of Senator Casey hoped for more from him and today are sadly disappointed."
Casey, a Roman Catholic from Scranton, opposes abortion rights except in cases of rape, incest or when the life of the mother is in danger.
Several other Democratic senators who, like Casey, represent swing states, have recently made similar announcements.
Sen. Rob Portman, a Republican from Ohio, announced his support for gay marriage in states that choose to allow it, saying his stance began to change in 2011, when his college-age son told him he was gay.