Ohio Senator Rob Portman has said that he feels comfortable talking about his endorsement of marriage equality.

Portman last year became the first sitting GOP senator to endorse such unions.

In an op-ed and later in interviews with CNN and the AP, Portman, a co-sponsor of the Defense of Marriage Act (DOMA), said that he had dropped his opposition after his son, Will Portman, told him and his wife Jane that he's gay.

Last week, the National Organization for Marriage (NOM), the nation's most vociferous opponent of marriage equality, announced that it will oppose Portman's reelection or possible presidential bid.

(Related: Gay marriage foe NOM vows to target Sen. Rob Portman for defeat in 2016.)

Portman told the AP that he's comfortable talking about the issue.

“Every week, sometimes every day, somebody will talk to me about it,” Portman said.

“I feel very comfortable in taking a position of respecting people for who they are,” he continued, “which is what I think ultimately same-sex marriage is about.”