Andrew Sullivan tells Fareed Zakaria at CNN why Catholics should be able to accept same-sex marriage:
I would say the religious arguments are more based in fear than in the actual teachings, that they're based upon stray texts that actually don't mean what you think they mean and that Jesus himself only said one thing about marriage, which is that you can't divorce. And we live in a country where countless people are divorced. And that doesn't seem to threaten the religious liberty of Catholics. And it's as fundamental an issue. So if Catholics can live with religious liberty with divorced people, they should be perfectly able to live with gay people, I mean, as married, as a civil marriage.
He also points Zakaria to Justice Kennedy's decision striking down DOMA because, as Kennedy repeatedly asserted, the law deliberately targeted a class of American citizens with the intent to rob them of human dignity. As Sullivan notes, the term "dignity" looms large in Catholic social teaching, and is a core, central value in that teaching.