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Rob Portman’s dual revelations that his son is gay and that he has decided to support gay marriage are both a touching story of familial love and another signpost in the astonishingly rapid success of the gay-rights revolution. Just over eight years ago, when Republicans gleefully seized on the gay-marriage issue to mobilize their base in Portman’s own state, it was inconceivable that a statewide Democrat would endorse gay marriage, let alone a Republican. The triumph of the issue relies upon the changing of minds — some thanks to force of argument, others to personal contact with gay friends, colleagues, and neighbors. From that standpoint, Portman’s conversion is a Very Good Thing.

And yet as a window into the working of Portman’s mind, his conversion is a confession of moral failure, one of which he appears unaware.

Here is the story Portman tells, in a Columbus Dispatch op-ed, of how he came to change his mind:

At the time, my position on marriage for same-sex couples was rooted in my faith tradition that marriage is a sacred bond between a man and a woman. Knowing that my son is gay prompted me to consider the issue from another perspective: that of a dad who wants all three of his kids to lead happy, meaningful lives with the people they love, a blessing Jane and I have shared for 26 years.

By Portman’s own account, in other words, he opposed gay marriage until he realized that opposition to gay marriage stands in the way of his own son’s happiness.

Wanting your children to be happy is the most natural human impulse. But our responsibility as political beings — and the special responsibility of those who hold political power — is to consider issues from a societal perspective.

It is possible to argue that the societal cost of granting the right to gay marriage — or, say, access to health insurance — outweighs the benefit. The signal failure of conservative thought is an inability to give any weight to the perspective of the disadvantaged. It’s one thing to argue that society can’t afford to provide all its citizens with access to health insurance. It’s quite another to dismiss the needs of the uninsured because the majority has insurance. In his acceptance speech at the Republican National Convention, Paul Ryan dismissed universal health insurance as “a new entitlement we didn’t even ask for.” The construction was so telling — “we” meant the majority who have access to regular medical care and would rather not subsidize those who don’t.

It is also possible to change your mind on any of these questions. I support the estate tax. If I discovered my children were due to inherit a fortune from a long lost relative, it’s possible that the experience would prompt me to change my mind. I’d like to think it wouldn’t. And if I did change my mind, I’d have some obligation to explain how I had learned something new in the process of suddenly becoming the father of wealthy heirs — estate planning is way more onerous than I thought! — rather than simply construct a new rationale to suit my newly discovered self-interest. If I simply declared that my children’s newfound wealth had given me a previously absent sympathy for the economic rights of the very rich, you would rightly question the value of my thinking on anything.

In President Obama’s interview explaining his reversal on gay marriage, he cited contact with gay friends, but also wrestled with the competing demands of gay happiness against the prerogative of those wedded to traditional practices. (“When I hear from them the pain they feel that somehow they are still considered — less than full citizens when it comes to — their legal rights — then — for me, I think it — it just has tipped the scales in that direction.”)

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: 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.

In a CNN interview, Dana Bash repeatedly prodded Portman to reconcile his previous opposition to gay rights (which extended not only to marriage but also to not getting fired for being gay). He repeatedly confessed that it all came down to his own family:

But you know, what happened to me is really personal. I mean, I hadn’t thought a lot about this issue. Again, my focus has been on other issues over my public policy career… What would Portman say to gay constituents who may be glad he’s changing his position on gay marriage, but also wondering why it took having a gay son to come around to supporting their rights? “Well, I would say that, you know, I’ve had a change of heart based on a personal experience. That’s certainly true,” he responded with a shoulder shrug. But he also repeated a reality. His policy focus has been almost exclusively on economic issues. “Now it’s different, you know. I hadn’t expected to be in this position. But I do think, you know, having spent a lot of time thinking about it and working through this issue personally that, you know, this is where I am, for reasons that are consistent with my political philosophy, including family values, including being a conservative who believes the family is a building block of society, so I’m comfortable there now.”

It’s pretty simple. Portman went along with his party’s opposition to gay marriage because it didn’t affect him. He thought about gay rights the way Paul Ryan thinks about health care. And he still obviously thinks about most issues the way Paul Ryan thinks about health care.

That Portman turns out to have a gay son is convenient for the gay-rights cause. But why should any of us come away from his conversion trusting that Portman is thinking on any issue about what’s good for all of us, rather than what’s good for himself and the people he knows?