Sen. Rob Portman's announcement that he supports same-sex marriage has the feel of a watershed moment. The Ohioan is one of the most prominent elected Republicans to ever endorse gay marriage, adding momentum to a cause that now seems all but irreversible. As Chris Cillizza at The Washington Post proclaims, "The political debate on gay marriage is effectively over."

However, while Portman's announcement was welcomed by gay rights groups, some liberals have voiced criticisms over the reasons he gave for changing his mind — namely, that his son is gay. "I've had a change of heart based on personal experience," he told CNN's Dana Bash. "I hadn't expected to be in this position."

Portman's story is a familiar one. Dick Cheney, who has a lesbian daughter, was famously an outlier on gay issues in the Bush administration, which used opposition to gay marriage to drum up the conservative vote in the 2004 election. As Andrew Sullivan writes at The Dish, "Some will wonder why Republicans only seem to get this question when they have a gay member of their family."

Critics describe Portman's announcement as morally flawed, since he seemed unable to feel empathy for gays until the issue hit home. "Portman ought to be able to recognize that, even if he changed his mind on gay marriage owing to personal experience, the logic stands irrespective of it," writes Jonathan Chait at New York. "Support for gay marriage would be right even if he didn't have a gay son. There's little sign that any such reasoning has crossed his mind."

Others are more forgiving. As Frank Bruni at The New York Times writes:

Those are great questions. Appropriate ones, too. But to a certain extent, they ignore human nature — the imperfections of it, the complexities of it — and they disregard how many people who support gay rights got to the place they now proudly inhabit.

Because they grew up in a society that has portrayed, and in many instances still portrays, homosexuality in negative, stereotypical terms, they needed to be educated. They had a journey to make. And in as many cases as not, that journey involved an example smack in front of them that discredited the stereotypes and dispelled the fear. Maybe a college roommate was gay. Maybe an admired colleague. Maybe a brother, a sister, a daughter. Maybe a son. [The New York Times]

In fact, acquaintance with a gay person is one of the most important factors in determining whether one supports same-sex marriage. According to a 2009 poll by Gallup, 49 percent of people who knew a gay person were in favor of same-sex marriage, compared with only 27 percent of those who didn't know a gay person. 

And Portman's political bravery — he could easily draw a primary challenge on this issue alone — shouldn't be discounted. Framing the issue as a personal one could help him continue in office, writes Josh Barro at Bloomberg:

The push from inside the family isn't just about bridging an empathy gap; it also helps Republican politicians who would like to support same-sex marriage bridge a political gap. Having a gay son will actually make it easier politically for Portman to support same-sex marriage. His opponents will temper their attacks on him for fear of being seen to attack his son; voters skeptical of same-sex marriage may still relate to Portman’s choice to stand up for his family. [Bloomberg]