He said so himself in an article in the San Francisco Chronicle. I have called the Catholic League myself many times to give him the opportunity to deny that he supported civil unions, but they never did. Here is the excerpt from the article and a link:
Conservative activist William Donohue, the president of the Catholic League for Religious and Civil Rights, says he believes Bush's proposal for a constitutional amendment is a "cultural tipping point" that will restore a "culture of restraint and decency."
The ban will not prevent the acceptance of gay culture, he acknowledges, nor will it prevent same-sex civil unions, which he now supports. What it will do, he says, is prevent those couples from actually saying they are married, even if they enjoy virtually the same rights.
"I don't like all-or-nothing approaches anymore," Donohue said.