The handsome pair walked the red carpet at the 2014 TrevorLIVE event in Los Angeles on Dec. 7, People magazine first reported.
"I looked at him and said, 'You know, once we walk this carpet, you're totally gay,'" Herndon, 52, quipped to People. "He looked at me and said 'Dude!' He has a great sense of humor."
The singer, who has been previously married and divorced to women twice, said, "I've done a lot of work around forgiveness with people that I've hurt and people I've not been honest with because of my sexuality."
But he hasn't ruled out tying the knot with Collum. He also said the couple, who are devout Christians, weren't concerned about how to reconcile their faith with their sexuality: "We get to choose who we love, and that includes God, and He loves us back."