Though he is a staunch conservative, Portman was never outspoken against gay marriage. But he consistently voted against it.

While in Congress, he supported a constitutional amendment to ban gay marriage, voted for the Defense of Marriage Act and voted for a bill prohibiting gay couples in Washington from adopting.

In 2011 100 University of Michigan law school graduates walked out of Portman's commencement address to protest his position on gay rights after circulating a petition trying to get him removed as the event's speaker.

"The decision to host a graduation speaker who is openly hostile to LGBT rights is deeply unfair to the LGBT students who will be in the audience this year celebrating their graduation," read the petition.

Portman admitted that when the protest occurred, he already knew his own son was gay.

"It was a little odd. Look, and to be honest with you, it didn't affect that decision-making much, because it was, I didn't think it was appropriate," Portman said. "But look, they had their freedom of speech rights to be able to do what they wanted to do."

"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," said Portman.

Asked about why he is announcing this change of heart, since his son revealed it to him two years ago, Portman cited two reasons. He just recently became comfortable with his decision to shift his position on gay marriage, and also he said he knew the Supreme Court is considering a pair of gay marriage cases, and reporters would likely ask him for his position.

"I thought it was the right time to let folks know where I stand so there's no confusion, so I would be clear about it," said Portman.

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."

To be sure, Portman was anything but comfortable discussing something as private and personal as his son's sexuality, even noting how foreign it felt for someone with his Midwestern upbringing and sensibilities to talk publicly about such issues.

At times, the press-savvy politician even seemed to tremble a bit. But it was also clear he was willing to endure the discussion in order to publicly tell his son that he is proud.

"He wanted to tell us that there's something about him that we didn't know," Portman said, reflecting on the day two years ago his son Will first told his parents he was gay.

Portman quickly added that it "of course hasn't changed our view of him at all."